One of the most notorious ancient practices that Christianity rebelled against was the frequent practice of expositio, basically the abandonment of unwanted infants. (Of course, girls were abandoned much more often than boys, which meant, as the historical sociologist Rodney Stark has pointed out, that Roman society had an extremely lopsided gender ratio, contributing to its violence and permanent tension.)
Another notorious practice in the ancient world was the sexual exploitation of children. It is sometimes pointed to paganism’s greater tolerance (though by no means full acceptance) of homosexuality than Christianity as evidence for its higher moral virtue. But this is to look at a very different world through distorting lenses. The key thing to understand about sexuality in the pagan world is the ever-present notion of concentric circles of worth. The ancient world did not have fewer taboos, it had different ones. Namely, most sexual acts were permissible, as long as they involved a person of higher status being active against or dominating a person of lower status. This meant that, according to all the evidence we have, the sexual abuse of children (particularly boys) was rife.
Think back on expositio. According to our sources, most abandoned children died — but some were “rescued,” almost inevitably into slavery. And the most profitable way for a small child slave to earn money was as a sex slave. Brothels specializing in child sex slaves, particularly boys, were established, legal, and thriving businesses in ancient Rome. One source reports that sex with castrated boys was regarded as a particular delicacy, and that foundlings were castrated as infants for that purpose.
Of course, the rich didn’t have to bother with brothels — they had all the rights to abuse their slaves (and even their children) as they pleased. And, again, this was perfectly licit. When Suetonius condemns Tiberius because he “taught children of the most tender years, whom he called his little fishes, to play between his legs while he was in his bath” and “those who had not yet been weaned, but were strong and hearty, he set at fellatio,” he is not writing with shock and horror; instead, he is essentially mocking the emperor for his lack of self-restraint and enjoying too much of a good thing.
This is the world into which Christianity came, condemning abortion and infanticide as loudly and as early as it could.
This is the world into which Christianity came, calling attention to children and ascribing special worth to them. Church leaders meditated on Jesus’ instruction to imitate children and proposed ways that Christians should look up to and become more like them.