Weird-ass pagan child worship located nowhere in the Bible. The strange idea that people are born clean and deserving, and that life makes them filthy and worthy of death. A really hateful, sadistic ideology. Christians did not care about abortion until the late 18th century, it's not even an old belief.

You can have a nearly identical baby in less than a year. A 50-year old takes 51 years to make.

That being said, the weird secular idea that babies are meaningless lumps of protein until they are born and are suddenly ensouled is equally silly. They're just arguing about dates, and at least the Christian tradition actually believes in a soul. I don't think most secular people could explain why you shouldn't kill an infant other than they find it distasteful, like eating a dog (when you eat pig and cow.)

That abortion is more distasteful than murder is a modern political culture-war position, though, that nobody would have considered until at least the early-mid 20th century. In fact, the idea that babies were sinless meant that they would go to Heaven, so 19th century Christians greatly preferred to murder a baby, turn themselves in and be executed for the crime rather than commit suicide (which was an express ticket to Hell.) Both the baby and the believer could then go to Heaven.