When I was an atheist*, I didn't want to believe. Heaven and Hell were idiotic and coercive, the concept of sin was a guilt trip, atonement by blood sacrifice was
literally paganism, the whole thing was about controlling people (especially women), and in the unlikely event that it was true, I wanted nothing to do with such a narcissistic, tyrannical god anyway.
All that said, I've kind of converted anyway.
Most of my problems came from having too simplistic an understanding of the religion and focusing on the wrong things. I still have some significant theological issues, but those are minor problems compared to hating absolutely
everything about the religion. It's impossible to believe if you don't
want to believe, and it's certainly possible to reject it unconsciously while thinking you're open to it. Which is most certainly not to say that
all disbelief is willful, but I definitely spent a good decade admiring Christianity while still being too intellectually proud to entertain the possibility that there might be something to it. I've probably still got some psychological barriers to authentic faith, but I'm trying to be cooperative about getting rid of them. Or I've gone completely delusional!
*I identified as an atheist because I thought organized religion was ridiculous. My problems were with the idea of divine revelation, anthropomorphic concepts of God, and the snarled nest of tortured logic that is a lot of theology. I was always at the very least an agnostic pantheist, so sincere atheists are going to have additional problems.