And perhaps more to the point, for most of human history it was essentially inconsequential whether anyone believed in any deities or not.In the West, the idea that religion was a matter of belief and not practice is a pretty recent phenomenon... it's basically a product of the Protestant Reformation.
Before that, the religiosity of a baby was a moot point: since a baby has no religious obligations, it can't fail to meet its religious obligations or refuse to perform them.
People who allowed such matters to actually affect their personal and social behavior in any discernible way were very few and far between. Nearly all people died too soon and were too busy with matters of a more concrete nature for any practical need to discern theists from atheists to truly exist.
Even today, it is often expected of atheists in Catholic families to simply avoid the matter, because it is not supposed to make much of a difference. It is my understanding that the same is in fact true of many other Christian and Muslim circles as well.