This is not entirely true. The concept of ensoulment has a history within the Church, but the condemnation of abortion goes back much further to the earliest Christian scholars in 100 A.D., who forbade "murder of a child by abortion" and any number of chemical concoctions intended to induce miscarriage. Anything intended to "destroy" rather than "create" was considered contrary to God's laws. We know this because they thought it was hilarious that they were accused of being cannibals while abortion was common in the Roman world. They argued against abortion because it involved murder, violence, selfishness, witchcraft, an indifference toward life, an indifference toward children, a misunderstanding of human biology ("that is a man which is going to be one; the fruit is already in the seed" - Tertullian), and a misunderstanding of your responsibility to your neighbor, which includes the child who is dependent upon you but not part of you. It was never based purely on the speculative theory of ensoulment, though that does appear sometimes in their treatises against it, and it's usually tied to conception, not a "delayed" form. Ensoulment is just a theory designed to explain a biblical incident where John the Baptist and Jesus "leapt for joy in their mother's wombs" when their moms met. Their personhood is already present there, reactive, emotive, spiritual, and integral to the Incarnation. Christian thought has always understood the unborn as their neighbors, as fully human and worthy of protection, and they used a similarly wide range of arguments that pro-lifers use today to talk, on the one hand, of the value of all human life, and on the other, the responsibility of a mother to her child even in the womb.