St. Augustine’s most important statement about abortion, as Germain Grisez shows in his superb 1970 study, Abortion: The Myths, the Realities, and the Arguments,  “occurs in a context where he is explaining his view that marriage is of itself good and that it uses sexual desire well—though such desire is not of itself good—for the procreation of children.”   I thus think it helpful, before presenting Augustine’s most important statement about abortion and considering his views on ensoulment, to describe that context in some detail.

Augustine’s understanding of marriage and sexual desire
Some of the Eastern Fathers (e.g., Gregory of Nyssa, John Chrysostom) thought that according to God’s plan, prior to the fall, human beings would have been generated asexually and not through sexual union. Augustine rejected this view, saying: "I do not see what could prevent there having been honorable marriage and an immaculate marriage bed in Paradise....God could have arranged that, without any restless burning of sexual desire...children would be born." 

In this and other texts Augustine clearly affirms that prior to the fall and the entrance of concupiscence into the human heart sexual desire was not bad. But here he also speaks of the "restless burning of sexual desire" and the "wild heat of passion." He thought that this "restless" burning that was needed, after the fall, for males to become sexually aroused is sexual desire infected by concupiscence, and such sexual desire is not good. Concupiscence, which exists in us as a result of Adam's sin, is not the same as original sin, which is wiped out by baptism. But concupiscence is caused by original sin and inclines us to sin, although it is not itself sin. Concupiscence is operative in human persons after the fall, and Augustine sees it as necessarily operative in sexual desire in marriage, where, however, it can be put to good use; it is an evil that spouses can use well.

Augustine taught that the marital act, when sought explicitly for the sake of procreation—which he called the “good of children” (bonum prolis)--is entirely holy and good. Since procreation is "the primary, natural, and legitimate purpose of marriage,"  it is evident that conjugal union chosen to serve this good is completely without fault; it is indeed a "chaste activity” (opus castum).

Augustine also taught that the marital act is wholly good and meritorious if one spouse consents to it in order to serve the “good of fidelity” (bonum fidei) and to help the spouse petitioning the act to alleviate sexual desire and avoid "fornication." His thought can be summed up thus: "In marriage, intercourse for the purpose of procreation has no fault attached to it; but intercourse for the purpose of satisfying concupiscence, provided it is with a spouse, is but a venial fault because of fidelity; adultery or fornication, however, is a mortal sin."

By "venial fault" Augustine meant the sort of fault to which all of us are daily susceptible and of which we cannot claim to be free without lying. It is the kind of sin forgiven when we say the Lord's prayer.  His thought here is quite sound. A marital act sought precisely to satisfy concupiscence is marred by a selfish kind of love. Note that according to Augustine a spouse who consents to the marital act in order to serve the good of fidelity and to help a spouse seeking it precisely to satisfy concupiscence is doing something perfectly good, even if there is no explicit intent to procreate. Again, surely a sound teaching. Where Augustine can be faulted is in not taking into consideration the possibility that both spouses could be seeking the marital act in order to express their love and fidelity, without having an explicit procreative intent. He held, as did Pope John Paul II in his Theology of the Body, that a spouse commits adultery who seeks genital union with his spouse, not because she or he is his or her spouse but simply because the individual is one who can satisfy his lust.  All this provides the context for Augustine’s most important statement on abortion.

Augustine’s teaching on abortion
Augustine’s most important text on abortion follows after he speaks of spouses who deliberately seek to prevent conception and who act, in his judgment, as adulterers and not as husbands and wives. He goes on to say:
Having advanced to this point, they are led on to expose the children that are born unwanted. For they hate to keep and bring up those they were so anxious not to have. And so when they inflict cruelty on their own offspring, whom they begot against their wills, a shadowy wickedness advances into a wickedness evident in the light of day; by obvious cruelty the concealed is convicted of shamefulness. Sometimes this lustful cruelty—or cruel lust—progresses to the point that they even obtain poisons for sterility; if these do not work, they somehow snuff out and destroy within the viscera the fetus that has been conceived. They wish their offspring to be cut off before it lives, or if it was already living in the uterus they want it to be killed before it is born.

As Grisez says, “Augustine condemns vigorously the whole spectrum of acts from birth prevention…through infanticide. He sees all of them as a continuum of acts motivated by a desire for sexual gratification without a commitment to the procreation of new life….Moreover…Augustine believed that even more than human life was at stake when abortion is committed. Man inseminates and woman conceives, ‘but that a fetus is conceived and born is a divine work, not a human one’ (Contra Julianum, V, 34).” 

Augustine on ensoulment
But when, according to Augustine, does human life begin?  He is not certain. In commenting on the Septuagint version of Exodus 21:22-23 he observes that the Mosaic law does not treat the accidental abortion of an “unformed” fetus as a homicide. He opines that the “unformed conceptus” might in some way he anitmated, i.e., that there might be a human life before there is a recognizably human fetus, but he was not certain that there was a living soul in a body lacking in senses, and an unformed fetus lacked senses. Elsewhere, considering the Resurrection, Augustine says he is inclined to think that all who have begun life will rise again, even if  they were not developed to the point of being “formed.” Here he evidently assumes that life precedes form and that this life is human in some way.

 
Moreover, when directly confronted with this question Augustine declares that not only does he not know but that he doubts the question can be surely answered. There seems to be no empirical test because no one remembers when he began life and the process has not been observed. He is certain that every man is created by God but he cannot presume to know the details of the way.

 
But whether the life generated is “ensouled” or not makes no difference with regard to abortion. For Augustine this is always gravely immoral, and is in fact a particularly heinous deed, an insult to the God who gives us life.