Christians themselves in some cases,
A higher QUALITY education for others.
Some ppl just logically contemplate it and realize that it contradicts logic in all possible ways.
Some ppl look at the 3 million gods who have come and gone, and see no distinction between ones validity and anothers.
Some ppl don't fall for the large # of members "logical argument".
Some ppl end up in careers that expose them to such high lvl science, weather its quantum physics or evolutionary biology, that they have a hard time doing the 9 to 5 of rationality, then turning the rational part of their brain off for church...
But unlike what some Christians here claim, it's not from reading the "atheists bibles" of Darwin & Dawkins. There are atheists who couldn't even tell u who Dawkins is, and I've still yet to read any of his work. Not because i don't like the guy, i just see no imminent need to read it. I already know what i don't believe. Lol.
There's a lot of reasons clearly, but #1 would probably have to be the judgemental nature.
Ask a Muslim whose left Islam, but not to specifically go Christian, theyl often list the judgemental nature as a top contributor... But persecuted groups like homosexuals simply die there.... So its even more .... Intense.