Supposing someone has come to believe that there is a God. Let us also suppose that he has come to believe that if it’s Christianity, it’s gotta be Catholicism.
So the question to remove the gap would be, “Why Christianity?” Of all the religions and belief systems in the world, what makes Christianity more real than the rest? For the truly faithful, why do you believe that the Truth is in Christianity and not in any other religions, some of which also make (equally?) plausible and sensible claims?