I say 50%, but I might be giving the human race more credit than is due. Much more than 50% of the human population is religious (regrettably), but many of those people are not presented with counter arguments or taught how to logically evaluate. Either way, the 50% figure isn't overly important, it might only be 20%.
However in my lifetime, I have never encountered an argument for religion, or a belief in god that was convincing. Every time there has been a logical fallacy or a false premise slipped into the argument, that the pro-religious party has been unable to notice.
These people are not stupid, I have sought to find the most intelligent arguments for religion, to check that I am not missing something, to check that I am not just researching biased information.
It doesn't make sense to me that these intelligent people are presenting these flawed arguments without realizing it. And I believe the answer to this perplexing puzzle is in psychology.
I seems to me that when engaging in a discussion with an intelligent pro-religious person, the conversation always expands to leave a convoluted trail of arguments and counter arguments and tangents, that by the time the conversation is concluded, it is impossible for the short term memory to recall all of the connections between arguments in the debate. Thus the party with flawed arguments, can't integrate the new information received into their belief system, and the original belief remains intact.
I also believe that educating children with these flawed arguments at a young age solidifies them deep in the brain, with many neural connections. Thus when debating these central ideas at an older age, it is much harder to break the illogical connections (have the party remember why Argument X is invalid) without constant repetition of the new information. Adding on the difficulty of a fast moving conversation where new information is only handled by the short term memory, and it is almost impossible to have the person remember why an argument was invalidated at a later time.
To this day I haven't found a single argument for god or religion that is convincing. I hope this article helps you to understand why debating religious people, or any party with a strong complex belief about any topic, is often unfruitful.