Why Believe in God?
Reason #4
Men of all cultures share a moral law. Where could that universal standard have come from?
(Indeed, when Gentiles, who do not have the law, do by nature things required by the law, they are a law for themselves, even though they do not have the law, since they show that the requirements of the law are written on their hearts, their consciences also bearing witness, and their thoughts now accusing, now even defending them.)
- Romans 2:14,15 (niv)
Men just naturally know if certain behaviors and beliefs are good or bad.
See CS Lewis for argument.