Every human being on planet earth has the capacity to know the difference between right and wrong, good and evil. This is something we inherited from Adam and Eve when they ate from the tree "of the knowledge of good, and Evil" Those who live a wicked life style know in their conscience that it is wrong, and if they continue going against their own conscience, God will give them over to a reprobate mind.
The only thing a Christian can do is live the life of God in front of them that they can see the light, and sense the Water of life flowing out from inside them. This is proclaiming the Gospel even louder than words.
We are living epistles known and read of all men.