I am personally an atheist that is quite firm in his opinion. This doesn't mean I don't like other religions: I have absolutely no problem if you are. I just don't think there is sufficient proof God exists (I presume this is the Christian God).
The only evidence that has been put forward in this age old debate is the bible. For all we know, it could have been some guy a few thousand years ago who was insane. Or it could be the word of a God.
The reason I don't personally believe the bible is because it states facts that were presumed at the time but have now been scientifically proved wrong. For example, the world being millions of years old and not being just 6,000 and being gay not being a choice. Fossils have been found that are millions of years old like dinosaurs and research with genes and psychology have discovered that being gay is not a choice.
But this doesn't mean God isn't real. It is possible. So yes, there is reasonable doubt.