Without getting into the question of whether denying God damns one irredeemably to hell, I'd like to respond to the question contained within Rocket's topic title.
He uses the word "know" in a clever construction that seems to imply that if one has come to "know" anything about science and one is an atheist, there can be no "return" to theism. It's a good point so long as it isn't taken too far.
If one has studied science to the point that one accepts such theories as big bang and evolution, I would suggest that one could not become a fundamentalist of any faith. However, spiritual experiences do seem to happen (at least according to those who've experienced them) so I should think that it would be quite possible that a knowledgeable atheist could become a theist of sorts.
I don't think it is possible to revert from a position of knowledge to a position of ignorance unless there is some mental disability at work. Therefore, if a knowledgeable atheist claims to have reverted to fundamentalism, that person is either mentally unstable or intellectually dishonest.