Abortion was legal and common in the ancient pagan Greco-Roman world. It was not practiced by the Jews except when a mother was unable to have birth after protracted labor (solved today by cesarean birth).
The early church inherited biblical, Jewish moral values, creating the “Judeo-Christian ethic.” The rapid spread of Christianity throughout the Roman empire during the 1st two centuries A.D. caused a widespread change in the pagan world’s attitude toward fetal life.
Consequently, abortion was made illegal under the secular Roman law at the beginning of the 3rd century. This was a side effect of Christian evangelism.
During the 4th century, many theologians became fascinated by Greek philosophy. As a result, Aristotle’s pagan view, that the baby was not “formed” and did not get its soul until sometime after conception, became popular again.
However, all of these Christian theologians still considered abortion at any stage of development as killing. A 4th-century church father, Basil of Caesarea, said it this way: “The hairsplitting difference between formed and unformed makes no difference to us. Whoever deliberately commits abortion is subject to the penalty for homicide.”