Despite the presence of religious ceremony in the Trilogy, there is no indication that Vito, his sons, or any other main characters believed in God or even acknowledge any religious adherence.
.....nor do we know anything of the contrary. My best guess is that the majority of the main characters were Catholic and basically believed in God but weren't too concerned with being what might be considered "Good Practicing Catholics", which is what many if not most Catholics could be considered today. Many Catholics today are what is called "Buffet Catholics", they pick and chose parts of the religion that appeals to them.
