What does Proverbs 20:27 mean?

Man's spirit possesses thought, motives, intentions, feelings, and desires, all of which the Lord tests. God is aware of every aspect of our inner lives (1 Samuel 16:7; Hebrews 4:13). A person's spirit illuminates who and what they really are (Luke 6:45). A person's spirit is seen by God; it shows everything needed to know to properly judge them (Psalm 7:11; Psalm 139:7). In biblical terminology, "light" is associated with goodness, as well as truth and knowledge. God has access to the "light" of a person's spirit: factual truth about their innermost being.

When Jesus was in Jerusalem for the Passover, He cleansed the temple by forcing out corrupt businessmen (John 2:13–17). He knew that their motives were impure. As a result, local religious leaders (John 2:18) questioned Him. Many people accepted Jesus' message because of the miraculous things He did (John 2:23). In that same passage, it was noted that Jesus knew what was happening in each person's spirit (John 2:24–25). Indeed, the Lord "knows the secrets of the heart" (Psalm 44:21).