Preist slaps crying baby in the face at baptism

[Rev. Rye]

Well, in Catholicism, baptism is generally held to be the one thing keeping the baby from eternal damnation, and, according to the Catechism, "The Church and the parents would deny a child the priceless grace of becoming a child of God were they not to confer Baptism shortly after birth." They may not have been immediately browbeaten into it, but they were probably conditioned into just going with it.

Because apparently some christians believe god will send babies to hell, unless some ritual is performed that they're too young to understand?

Yes. The doctrine is called "Infant Damnation." To be fair, Catholicism (at least the mainstream kind) isn't comfortable with the idea of God sending babies that young to Hell, and the Catechism's official statement is they generally hope God does the right thing. Honestly, I think it's as much an initiation ritual as anything else.