The apostle Paul notes that “the god of this world [i.e., Satan] has blinded the minds of the unbelieving so that they might not see the light of the gospel.” Scripture also talks often about how those without the truth are walking in darkness. C. S. Lewis picks up on this theme in Mere Christianity when he explains how sin warps our understanding of our very sinfulness:

Good & EvilThe right direction leads not only to peace but to knowledge. When a man is getting better, he understands more and more clearly the evil that is still left in him. When a man is getting worse, he understands his own badness less and less. A moderately bad man knows he is not very good; a thoroughly bad man thinks he is all right.

This is common sense, really. You understand sleep when you are awake, not while you are sleeping. . . . You can understand the nature of drunkenness when you are sober, not when you are drunk. Good people know about both good and evil; bad people do not know about either.

I think our goal is spelled out in the book of Hebrews:

For everyone who partakes only of milk is not accustomed to the word of righteousness, for he is an infant. But solid food is for the mature, who because of practice have their senses trained to discern good and evil.

We need discernment. Only by growing in righteousness will we ever see clearly the distinction between good and evil.