There is in man that which longs to correspond with God- a God-shaped vacuum- a senses of oughtness- a realization of guilt.
From the life of Paul we can give a good illustration: When they were shipwrecked and they were building a fire, a snake came out and bit Paul on the hand. The pagan natives concluded that he was being punished for some crime . How did they gain the knowledge that sin deserves punishment?
Missionaries universally testify that they find no athetist in even the most primitive tribal setting.