Apologies if this has been asked before, but I searched and only found one unanswered post.
It's my understanding that the idea of life beginning at conception is a relatively recent one, especially since until recently, no one even really knew exactly what conceiving a child actually meant, in the sense of a sperm cell merging with an egg, which develops into a fetus. According to this article, the bible only mentions a human soul entering their body when the child draws its first breath, which would suggest that babies weren't even fully considered human until they were born. It also mentions verses where a woman is explicitly given an abortion remedy, which is framed as a good thing. As well, this article talks about how Christians in medieval times had complex and fluid views on abortion, but never considered it to be murder, exactly, with some venerated saints even having supposedly performed miraculous abortions.
I'm aware that until a few decades ago, the question of abortion was called the "Catholic problem," as protestants didn't really have firm views on it even then, but my question, I guess, is when did even the Catholics start preaching against abortion, and when did they propose the idea of life beginning at conception, and with what evidence and/or reason?
You are correct in pointing out the existence of varying interpretations and philosophical arguments relating to the moral permissibility of abortion throughout Catholic history. In addition to Augustine's writings mentioned in your linked article, Aquinas and some other medieval Catholic philosophers (in part due to their Aristotelian-informed approach to "natural philosophy" (i.e. the sort of intellectual investigation more in-line with what we think of today as science)) often spoke of "ensoulment" as an event distinct in occurrence from conception. Although, even this distinction is complicated by the fact that, as you mention, "conception" was obviously not understood with anywhere near the level of biological detail that we have knowledge of today. Additionally, Aquinas' view is even further complicated because, in spite of the fact the he believed that ensoulment occurred at some time after conception (40 days for boys, 80 days for girls), he nonetheless opposed acts that would intentionally end a pregnancy prior to the time of ensoulment. Haldane and Lee have a relevant paper that provides a good general overview of some of the philosophical and theological lineage of these discussions in Catholic history, and they go into much greater detail regarding Aquinas' notions of ensoulment in particular (see Haldane, J., & Lee, P. (2003). "Aquinas on Human Ensoulment, Abortion and the Value of Life." Philosophy, 78(2), 255-278.)
With regard to abortion generally speaking, it certainly isn't the case that opposition to abortion can be reasonably thought of as "new" phenomenon in Catholic history. Something important to remember (and that Allen neglects to some degree in the article you link) is that the existence of debate surrounding the moral treatment of abortion doesn't somehow imply that the "opposition" position wasn't really there. Though this paper is somewhat dated (a fact that may actually drive home this point more effectively given the framing of your question), John Noonan's 1967 "Abortion and the Catholic Church" plainly demonstrates the existence of fairly resolute opposition to abortion even in the first century. Considered by most scholars to have been composed toward the end of the first century AD, the Didache represents clear opposition to abortion in the earliest years of Christianity:
“The second commandment of the teaching: You shall not murder. You shall not commit adultery. You shall not seduce boys. You shall not commit fornication. You shall not steal. You shall not practice magic. You shall not use potions. You shall not procure [an] abortion, nor destroy a newborn child” (Didache 2:1–2).
There are troves of early Christian writings that explicitly oppose abortion in a similar manner- but of course, this this still begs the question of potential distinctions that could have been made between opposition to abortion and the assumption that life begins (and thus, could theoretically be aborted) at conception. But someone like Tertullian, as can been seen in his Treatise on The Soul (written in the late 1st or early 2nd century) leaves little room for debate:
“Now we allow that life begins with conception because we contend that the soul also begins from conception; life taking its commencement at the same moment and place that the soul does ” (Treatise on The Soul, 27)
St. Jerome, writing toward the end of the fourth or early fifth century, provides similar (though perhaps slightly less on-the-nose) evidence of the belief that life began at conception:
"You may see many women widows before wedded, who try to conceal their miserable fall by a lying garb. Unless they are betrayed by swelling wombs or by the crying of their infants, they walk abroad with tripping feet and heads in the air. Some go so far as to take potions, that they may insure barrenness, and thus murder human beings almost before their conception. Some, when they find themselves with child through their sin, use drugs to procure abortion, and when (as often happens) they die with their offspring, they enter the lower world laden with the guilt not only of adultery against Christ but also of suicide and child murder." (Letters 22:13)
These writings certainly don't imply that debate regarding the question of "ensoulment" in Catholic thought didn't continue (as mentioned earlier, it clearly did). But it does seem clear that from very early in Church history, the belief that life began at conception (and thus, that abortion was morally wrong) was certainly present and held by fairly influential figures.
I'll finish my response here for now. In case you find it helpful, I'll leave a link to an earlier post I made in response to a somewhat similar question- my response there might be of interest if you are wondering about more contemporary features of Christian views on abortion.