They don’t instantly lose all ability to discern good from evil and truth from error, but they lose a little bit with each resistance. Jesus is still the light of the world Who lights every person who comes into the world. We, who follow Christ, actually go back a bit whenever we sin. It hurts our relationship until we repent. For those who continually reject the leading of Christ and refuse to thank Him and glorify Him, they will come to a point where God turns fully from them and stops giving them light. They will begin to call evil good and good evil.
direct link – http://RealReality.org/Real_Faith_and_Reason_Vol_1_-_Scientia.pdf