Everyone who believes or disbelieves in God does so because of three factors

http://www.corningmennonite.org/tools/keller.html

• intellectual (rational, logical arguments for or against the existence of God);

• personal (experiences and how they are interpreted);

• social (the beliefs of people we need or admire seem more plausible to us than beliefs of people we don't need or admire).

Belief or disbelief should never be reduced to one of these three (eg. "you wouldn't be a Christian if you were born in Madagascar; but I used intellect and logic to arrive at my position.") No, all beliefs, whether secular or religious, have social as well as intellectual roots. All three of these factors play a part in disbelief as well as belief.