Christianity has roots going back to Abraham, and beyond. So I'd still so the Christian God. Besides, I didn't say that popularity is the only or even the biggest reason why Christianity can be considered the true religion. But it's perfectly logical that the creator of the world would have the largest following, as opposed to gods that don't exist.
That's assuming that
any of them exist. And even then, to assume that the "real" one has the more followers is ridiculous. Popularity can't be considered reason for anything to be the "One true religion", because every religion out there is a true religion.
Being the most popular doesn't make the validity of your beliefs any more reasonable than the beliefs held by Scientology.
It's the fastest growing religon, but NOT the largest:
Major Religions Ranked by Size
Chrisitianity is still easily the largest religion.
No, Islam isn't the largest, but I highly doubt it's as far off as you'd think. It's a lot easier to count heads in Westernised civilisation, where Christianity is predominant, than somewhere like the Afghanistan or Iraq, or anywhere else in the Middle East, for that matter, where Islam is more prevalent. The rate it
is growing, I doubt we'd have to wait 200 years, as Paltoall said, until Islam overtakes Christianity. Maybe not in "official statistics" at first, because they're only what's recordable, but it'd catch up with that, eventually.
And yes, your reasoning is illogical and highly flawed. What about the most popular religions of say... 10,000 years ago? Would the most popular religion then be correct?
One interesting thing to note is atheism being the third most popular "religion", with 1.1 billion, half of all followers of Christianity. 500 years ago if you were to've asked this, we wouldn't've even numbered enough to make it on the list.