Without addressing the actual question in the OP, I would like to address a kind of side-issue, and that is the logical fallacy implicit in the question itself. If the question had been, “What makes you think Christianity is true?” that would have been a straightforward question with no implied logical fallacy, and one might proceed to try to answer it in a straightforward manner. But when coupled with the first sentence about the number of religions, the question becomes an implied argument, and that implied argument is the one with the logical fallacy.
The implied argument goes like this: There are a large number of religions practiced in the world. They should all have the same probability of being true. The sheer number of them makes it harder to show that any one of them is completely true. Since Christianity is one of many religions, it has a small chance of being true.
The fallacy in that argument is in the statement: “They should all have the same probability of being true.” There is no reason a priori to assign every religion an equal probability of being true. The question of whether Christianity is true ought to be the same even if Christianity were the only religion that was practiced. In fact, if you accept the principles of Christianity, it makes perfect sense that there would be many religions. One of the premises of Christianity is the Man was created by God with an instinctive desire to seek him and to worship him. So when people spread around the world and became separated, even if their culture did not remember God, they would still have that instinct in them. Therefore they would invent religions in an effort to fulfill that desire. Even those religions, like Buddhism, that do not have an actual deity, still search for things like idealized truth and goodness - characteristics we associate with the God of Christianity.
Of course that explanation could be offered by just about every other religion too, so it is not a complete answer to the OP question. But it does at least minimize the importance of there being so many religions in challenging the true of Christianity.
So I suggest that people trying to answer the OP question instead answer the question “What makes you think Christianity is true?” and forget about the sentence before it.