why do you follow religion A? accident of birth: you were born in a country where most people follow A. and so, you are taught and are 'sure' that A is true.
if you were born in country B, you'd be 'sure' that B's religion is true and factual.
I wish I had mod points to use here. I had this same realization many years ago, and soon after I abandoned my religion of birth (Christiantiy) and decided to pursue science. Unfortunately this argument doesn't work well on most religious people since they can come up with a handful of names that converted to their religion, which they use to argue that a truly enlightened person will find his way to the "correct" religion. Of course, they never mention anyone who converts in the other direction...
"This generation may be the one that will face Armageddon." -- Ronald Reagan, "People" magazine, December 26, 1985