I am going to give four different arguments for this position. The first two arguments draw from Judeo-Christian history; the others will offer arguments that can be made apart from a belief in the veracity of the Bible.
1. THE ARGUMENT FROM SCRIPTURE
Historically, the Judeo-Christian consensus has been that the unborn child is a human being.
- "Did not He who made me in the womb make him, and the same one fashion us in the womb? (Job 31:15)
- “For You shaped me, inside and out. You knitted me together in my mother’s womb long before I took my first breath. I will offer You my grateful heart, for I am Your unique creation, filled with wonder and awe. You have approached even the smallest details with excellence; Your works are wonderful; I carry this knowledge deep within my soul. You see all things; nothing about me was hidden from You As I took shape in secret, carefully crafted in the heart of the earth before I was born from its womb. You see all things; You saw me growing, changing in my mother’s womb…”(Psalm 139:13-16
- “But now hear, O Jacob my servant, Israel whom I have chosen! Thus says the Lord who made you, who formed you from the womb and will help you: Fear not…” (Isaiah 44:2)
- “…the LORD, your Redeemer, and the one who formed you before birth…’” (Isaiah 44:24)
- The Eternal One singled me out, even before I was born. He called me and named me when I was still in my mother’s belly. Even then, God was preparing my mouth to speak like a sharp sword... And now the Eternal who watched, shaped, and made me His own servant from the womb has determined to restore Jacob’s family…” (Isaiah 49:1,5)
- "Before I even formed you in your mother’s womb, I knew all about you. Before you drew your first breath, I had already chosen you to be My prophet to speak My word to the nations." (Jeremiah 1:5)
In other words, the penalty for the harm experienced by both mother and child is the same."And if men struggle with each other and strike a woman with child so that the child comes forth, yet there is no injury [premature birth], he shall surely be fined as the woman's husband may demand of him; and he shall pay as the judges decide. But if there is any injury [to the woman or the miscarried baby], then you shall appoint as a penalty life for life, eye for eye, tooth for tooth, hand for hand, foot for foot, burn for burn, wound for wound, bruise for bruise...." (Exodus 21:22-25)
Once again, we don’t see a clear injunction in the New Testament against elective abortion, most likely because the books in the New Testament were written by Jewish people coming out of a culture where abortion simply was not done. However, the humanity of the unborn is once again supported.
- Of John the Baptist: "For he will be great in the sight of the Lord, and he will drink no wine or liquor; and he will be filled with the Holy Spirit, while yet in his mother's womb." (Luke 1:15)
- “At the sound of Mary’s greeting, Elizabeth’s child leaped within her, and Elizabeth was filled with the Holy Spirit.” (Luke 1:41) The Greek word “babe,” brephos, is used equally of an unborn child and an infant (see Luke 2:12, 16; Acts 7:19).
- Paul talked about God, “He who had set me apart, even before I was born, and called me through His grace…” (Galatians 1:15)
THE ARGUMENT FROM CHURCH HISTORY
In Judaism, there was a consensus on a number of important things:
- people were made in God’s image
- children were a blessing (so elective abortion was unthinkable)
- the unborn were humans deserving of protection
In spite of this uncertainty, there were clear teachings about how seriously they viewed the issue. Josephus (a first-century Jewish historian) stated, “The law orders all the offspring be brought up, and forbids women either to cause abortion or to make away with the fetus.” A woman who did so was considered to have committed infanticide because she destroyed a “soul” and hence diminished the race.
When specifically addressing the issue of abortion, the early Church built on the foundation already in place and unhesitatingly condemned abortion as the killing of an innocent person. The church was now drawing Greek and Roman converts, people for whom abortion and infanticide were considered regular aspects of life, and we see the stated position of the church clarifying as history unfolds.
- The epistle of Barnabas (approx. 125): "Thou shalt not slay the child by procuring abortion; nor, again, shalt thou destroy it after it is born."
- The Didache (approx. 140): " Do not murder a child by abortion, nor commit infanticide.”
- Athenagoras wrote A Plea for the Christians (approx. 177) to debunk the charge that Christians kill infants during their worship services in order to eat their flesh and drink their blood. “And when we say that those women who use drugs to bring on abortion commit murder, and will have to give an account to God for the abortion, on what principle should we commit murder? For it does not belong to the same person to regard the very fetus in the womb as a created being, and therefore an object of God’s care, and when it has passed into life, to kill it; and not to expose an infant, because those who expose them are chargeable with child-murder, and on the other hand, when it has been reared to destroy it.”
- Clement of Alexandria, in The Tutor (approx. 200): “Those who use abortifacient medicines to hide their fornication are causing the outright destruction, together with the fetus, of the whole human race.”
- Hippolytus of Rome, in Refutation of All Heresies (approx. 210): “Whence women, reputed believers, began to resort to drugs for producing sterility, and to gird themselves round, so to expel what was being conceived on account of their not wishing to have a child either by a slave or by any paltry fellow, for the sake of their family or excessive wealth. Behold into how great impiety that lawless one has proceeded by inculcating adultery and murder at the same time!”
- Tertullian, in Apologetics (approx. 213): " It makes no difference whether one take away the life once born or destroy it as it comes to birth. He is a man who is to be a man; the fruit is always present in the seed.”
- The Council of Ancyra in A.D. 315 decreed: “Concerning women who commit fornication and destroy that which they have conceived or who are employed in making drugs for abortion, a former decree excluded them [from Communion] until the hour of death and to this some have assented. Nevertheless, we have ordained that they fulfill ten years [of penance], according to the prescribed degrees.”
- Basil the Great, in Letter to Amphilochius (approx. 360): “The woman who purposely destroys her unborn child is guilty of murder. With us there is no nice enquiry as to its being formed or unformed. . .”
- Gregory of Nyssa, On the Soul and Resurrection (approx. 370): “But there is no disagreement or doubt that those which are being nourished in the womb have growth and spatial movement. So the remaining alternative is to suppose that soul and body have one and the same beginning.”
- Chrysostom, in a homily on Romans (approx. 390): “Why sow where the ground makes it its care to destroy the fruit? Where there are many efforts at abortion? Where there is murder before the birth?”
- Jerome, in Letter to Eustochium (approx. 400): “Some, when they find themselves with child through their sin, use drugs to procure abortion,” which he called “child murder.”
- Sixth Ecumenical Council (in Trullo) in 680: “Those who give drugs procuring abortion and those who receive poisons to kill the fetus are subjected to the penalty of murder.”
THE ARGUMENT FROM BIOLOGY
I’ve talked before about how the revelation of the Bible coincides with God’s revelation through His creation. In other words, if all truth is God’s truth, we would expect the Bible and the world, when both are properly understood, to give the same message. In the case of the unborn, we see science and biology bringing clarity to the question of life. There is a clear consensus that life begins at conception.
- "It is the penetration of the ovum by a spermatozoan and resultant mingling of the nuclear material that each brings to the union that constitutes the culmination of the process of fertilization and marks the initiation of the life of a new individual." (Bradley M. Patten, Human Embryology, 3rd ed., New York: McGraw Hill, 1968, page 43.)
- "Every time a sperm cell and ovum unite a new being is created which is alive and will continue to live unless its death is brought about by some specific condition." (E. L. Potter and J. M. Craig, Pathology of the Fetus and the Infant, 3rd ed., Chicago: Year Book Medical Publishers, 1975, page vii.)
- The beginning of a single human life is from a biological point of view a simple and straightforward matter - the beginning is conception." (Subcommittee on Separation of Powers to Senate Judiciary Committee S-158, Report, 97th Congress, 1st Session, 1981.)
- "Physicians, biologists, and other scientists agree that conception marks the beginning of the life of a human being - a being that is alive and is a member of the human species. There is overwhelming agreement on this point in countless medical, biological, and scientific writings." (Subcommittee on Separation of Powers, Ibid.)
- “The science of embryology is clear. From the earliest stages of development, the unborn are distinct, living, and whole human beings. Therefore, every ‘successful’ abortion ends the life of a living human being.” (Scott Klusendorf, The Case for Life, Crossway Books, 2009, p. 35.)
- “Although life is a continuous process, fertilization (which, incidentally, is not a ‘moment’) is a critical landmark because, under ordinary circumstances, a new genetically distinct human organism is formed when the chromosomes of the male and female pronuclei blend in the oocyte.” (Ronan O’Rahilly and Fabiola Muller, Human Embryology and Teratology, 3rd ed., New York: Wiley-Liss, 2001, p.8.)
- “A zygote is the beginning of a new human being (i.e., an embryo).” (Keith L. Moore, The Developing Human: Clinically Oriented Embryology, 7th ed., Philadelphia, PA: Saunders, 2003, p.2.)
- “It is possible to give ‘human being’ a precise meaning. We can use it as equivalent to ‘member of the species Homo sapiens.’ Whether a being is a member of a given species is something that can be determined scientifically, by an examination of the nature of the chromosomes in the cells of living organisms. In this sense there is no doubt that from the first moments of its existence an embryo conceived from human sperm and eggs is a human being.” (Peter Singer, Practical Ethics, 2nd ed., Cambridge: Cambridge University Press, 1993, pp.85-86.)
- Dr. Warren Hern, author of Abortion Practice, told Planned Parenthood: “We have reached a point in this particular technology [D&E abortion] where there is no possibility of denying an act of destruction. It is before one’s eyes. The sensations of dismemberment flow through the forceps like an electric current.”
- In 1970, an editorial in California Medicine noted: “Since the old ethic has not yet been fully displaced it has been necessary to separate the idea of abortion from the idea of killing, which continues to be socially abhorrent. The result has been a curious avoidance of the scientific fact, which everyone really knows, that human life begins at conception and is continuous whether intra-or extra-uterine until death. The very considerable semantic gymnastics which are required to rationalize abortion as anything but taking a human life would be ludicrous if they were not often put forth under socially impeccable auspices. It is suggested that this schizophrenic sort of subterfuge is necessary because while a new ethic is being accepted the old one has not yet been rejected.”
- Former Planned Parenthood President Dr. Alan Guttmacher was perplexed that anyone would question these basic scientific facts. "This all seems so simple and evident that it is difficult to picture a time when it wasn't part of the common knowledge," he wrote in his book Life in the Making. (A. Guttmacher, Life in the Making: The Story of Human Procreation, New York: Viking Press, 1933, p. 3.)
- A Planned Parenthood brochure in 1963 noted, “Abortion kills the life of a baby after it has begun. It is dangerous to your life and health.”[i]
- Faye Wattleton, the longest reigning president Planned Parenthood, told Ms. Magazine: “I think we have deluded ourselves into believing that people don't know that abortion is killing. So any pretense that abortion is not killing is a signal of our ambivalence, a signal that we cannot say yes, it kills a fetus.”
- Bernard Nathanson co-founder of NARAL, in an article for the New England Journal of Medicine in 1974: "There is no longer serious doubt in my mind that human life exists within the womb from the very onset of pregnancy..."
THE ARGUMENT FROM PHILOSOPHY
Philosophy supports the Biblical narrative, historical Christian position, and biology Scott Klusendorff (Life Training Institute) and Greg Koukl (Stand To Reason) have highlighted the four ways in which an unborn child differs from one who is born (Size, Level of development, Environment, and Degree of dependency), none of which justifies the elective killing the unborn.
The unborn is clearly smaller than a born human. This does not mean they are not a person. Three-year-olds are smaller than a teenagers. Can we kill them? Our value is not based on our size. In the same way, though the unborn is smaller than a born child, this is not a justifiable reason for killing the unborn.
Level of development
The unborn is also less developed than a born human being, but this is irrelevant to personhood. A four year-old girl can’t bear children because her reproductive system is less developed than a fourteen year-old girl. She is still as equally valuable as a child-bearing teen – or a seventy-year old grandmother. We can’t disqualify the unborn from personhood simply because they are less developed than older human beings, and this includes going back to the most fundamental starting point of our development.
Another way of thinking of this is asking the question: Were you ever not you? Of course not. If you were to walk backward through your life history, you would walk back to the moment of your conception.
You location has no bearing on the value of who you are. Being inside or outside a house changes nothing; being inside our atmosphere or out of it does not change an astronaut’s humanity. In the same way, a journey from inside the womb to outside the womb changes nothing about one’s humanity or personhood. If you are a person when you are born, you were a person the moment before that too.
Even Peter Singer agrees with this. Singer, an ethicist at Princeton, argues that infanticide should be legal for at least a month after birth. That’s a horrible but consistent idea for him. He wrote in his book Practical Ethics:“The liberal search for a morally crucial dividing line between the newborn baby and the fetus has failed to yield any event or stage of development that can bear the weight of separating those with a right to life from those who lack such a right.” 
I agree. To Singer, that means if we can kill the unborn we can kill the born. He badly misses the point. If we can’t kill the born, we shouldn’t kill the unborn either.
Degree of dependency
Sometimes people cite ‘viability’ as a marker for when the unborn should be considered human. But newborns and toddlers are hardly viable in the truest sense of the world. They must be fed and cared for by someone else. Is Peter Singer right? Should parents be allowed to kill children until they are independent in terms of their need for basic sustenance? No. Your humanity is not connected to your dependency. If it were, people in hospitals would be less human. People at the end of life would be less human. People who are handicapped physically or mentally would be less human. Yes, the unborn depends on her mother, but this says nothing about their humanity and value.[ii]
This SLED acronym actually speaks to a broader pro-life position: no human being – regardless of size, level of development, place of residence or degree of dependence – should be excluded from the community of human persons and the rights and protections that follow.
NOTE: I did not reference any “hard cases” such as rape and incest, serious medical difficulties in the baby, or times the mother’s life is in danger. These are situations that need to be answered carefully, compassionately and truthfully, but they are broader than the purposes of this article.
- The Case for Life. http://www.caseforlife.com
- The Bible and Abortion. http://www.godandscience.org/doctrine/prolife.html
- Argument for the Silent: A Biblical Case Against Abortion. http://www.reasons.org/articles/argument-for-the-silent-a-biblical-case-against-abortion
- Abortion and the Early Christian Church. http://www.godandscience.org/abortion/earlychristian.html
- The S.L.E.D. Test. http://www.str.org/articles/the-s.l.e.d.-test#.VktEzkvM-H0
- Life News. lifenews.com
- “Five Bad Ways To Argue About Abortion.” http://prolifetraining.com/resources/five-bad-ways/
- AfterAbortion.org. http://afterabortion.org/help-healing/
 “That an Israelite parent might consider intentionally aborting a foetus seems almost beyond the moral horizon of the Torah's original audience. For in the moral environment where the law was first received, the memory of genocide and infanticide was still fresh [and] every birth was precious.” Lenn E. Goodman, Judaism, Human Rights, and Human Values, OUP 1998
 “What Exodus 21:22 Says about Abortion.” Stand To Reason. Some think this just refers to a punishment if the mother dies or is injured. This article makes a clear and compelling argument that the punishment applied equally if either the mother or the child was the victim.
 “Dead Silence: Must The Bible Say Abortion Is Wrong Before We Can Know It’s Wrong?”
 The State of Israel is now worlds away from the Old Testament perspective. See “Israel’s abortion law now among world’s most liberal.”
 The Early Church on Abortion. http://www.orthodoxytoday.org/articles/EarlyChurchAbortion.php
 The list so far was taken from “Even “Pro-Choice” Philosophers Admit: Human Life Begins at Fertilization.”
 “A new human being comes into existence during the process of fertilization.”
 “Peter Singer’s Bold Defense of Infanticide.”
 “The Hard Cases Objection: Does Rape Justify Abortion?”
 “Is Abortion permissible for Fetal Deformity?”
 “Is Abortion Justifiable In The Hard Cases?”