Oh boy… The duration of belief has no bearing on whether or not its true. A claim is either as false or true today as it was then. Simply because it stays in existence does not mean it is true. Many other myths have also been passed down. But by your standard of evidence, I’ll go ahead an remind you of the other large religions that have been passed down even longer. An atheist would not deny that the catholic church or any other does not exist. Rather, the argument would be that the actual beliefs are not true. That this Church has remained in tact all these long centuries is impressively unique. You have only answered in general …never naming any religions that come close to its longevity. In other words, this fact would/should give it enough gravity to cause one to look more closely to discover why this is true. Also to look closer at its Founder and His claims. You have not named another historical figure that can even come close to Christ in terms of the loyalty and faithfulness of His followers that continues today in as large numbers. Again this is not proof in itself but should prompt further investigation on the part of the sincere seeker. At that point the seeker would begin in all seriousness to read some of the written evidence given throughout history (the books I have mentioned ) Some of them historians such as Pliny, Josephous, as well as the more recent books written by Benedict XVI on Jesus of Nazareth, filled with more references than I can give here. One important point among many others he makes is that it is important to distiquish between the historical Jesus and the Christ of faith…the historical Jesus cannot and does not explain the Jesus as He was in real history!
Why not read the Qu’ran? Because we are addressing the historical Jesus and the Christ of faith it would not be appropriate to select a book about the beliefs promoted about him by Mohammed…who while admitting to His existence does not recognize Him in His fullness as well as the fact that the Qu’ran must necessarily “distort” Biblical truth as well as deny much of it.
Some atheists may deny his existence, but I don’t. It’s very likely he was a real man claiming to be divine. But the fact that he may have been a good magician in his time and gained followers is not sufficient to prove he is the creator of the universe.