Actually, what put the Christians and Romans on a collision course is not that Christians believed in a God that cared about belief as opposed to participation in public rituals, but that Christians believed rather strongly in a God that cared about participation in public rituals, and, particularly, that Christian belief conflicted with participation in the public rituals of the Roman state religion, which while it might not care about belief, did care very much about everyone subject to it participating in its public ritual.