Numbers are a tempting argument when one’s side is in the majority, but they simply do not work as any sort of proof of the correctness of a faith system.
When you argue from the “there are more of us so we must be right” position, you leave yourself open to the corollary—that if someone else either at one time had more supporters or gains additional supporters in the future, then
they must have been or will be right and you wrong, simply because they have more numbers.
See if this makes it clearer
nizkor.org/features/fallacies/appeal-to-popularity.html
nizkor.org/features/fallacies/appeal-to-common-practice.html
csun.edu/~dgw61315/fallacies.html#Argumentum%20ad%20numerum
*Argumentum ad numerum (argument or appeal to numbers). This fallacy is the attempt to prove something by showing how many people think that it’s true. But no matter how many people believe something, that doesn’t necessarily make it true or right. Example: “At least 70% of all Americans support restrictions on access to abortions.” Well, maybe 70% of Americans are wrong!
This fallacy is very similar to argumentum ad populum, the appeal to the people or to popularity. When a distinction is made between the two, ad populum is construed narrowly to designate an appeal to the opinions of people in the immediate vicinity, perhaps in hope of getting others (such as judges) to jump on the bandwagon, whereas ad numerum is used to designate appeals based purely on the number of people who hold a particular belief. The distinction is a fine one, and in general the terms can be used interchangeably in debate rounds. (I’ve found that ad populum has better rhetorical effect.)
Argumentum ad populum (argument or appeal to the public). This is the fallacy of trying to prove something by showing that the public agrees with you. For an example, see above. This fallacy is nearly identical to argumentum ad numerum, which you should see for more details. *
So if you base your argument that Christianity is the one true religion on the fact that there are more Christians than members of other religions, then the next logical questions are “at what point in its history did Christianity gain enough followers to become more true than other religions?” as well as “how many more followers does a religion need than its competitors to be the one true religion–one, five, five hundred, five thousand?” What happens if that margin narrows?
It is a very weak argument and you would do better to find a stronger basis for belief than mere popularity.