"Wherefore be ye not unwise, but understanding what the will of the Lord is." (Ephesians 5:17)
The "wherefore" is preceded by the command "Walk as children of light" (Ephesians 5:8). This title, children of light, is used only three other times in the New Testament: once by the Lord Jesus to contrast worldly wisdom with the ineffectual use of godly wisdom in the least things (Luke 16:8); once again to direct us to believe in the light (John 12:36); and finally by Paul to encourage us to watch and be sober (1 Thessalonians 5:5-6).
A light-like life, which is evidence of the fruit of the Spirit, is expressed in the character of goodness (Romans 15:14), righteousness (Romans 14:17-18), and truth (Ephesians 5:9; compare Galatians 5:22). In fact, the transformation of our character by our conscious choice to "present |our| bodies a living sacrifice, holy, acceptable unto God," enables us to "prove what is that good and acceptable, and perfect will of God" (Romans 12:1-2; see also Ephesians 5:10). An equation is clearly drawn between godly behavior and godly wisdom.
It therefore follows that children of light "should have no fellowship with the unfruitful works of darkness" (Ephesians 5:11), taking the responsibility to reprove them and recognizing the "shame even to speak of those things" (Ephesians 5:12).
The light things make manifest (present, display) that which is reproved, enabling us to "walk circumspectly |accurately, carefully|, not as fools but as wise" (Ephesians 5:15). That wisdom is not the foolish wisdom of this world (1 Corinthians 1:20), but the wisdom of God (1 Corinthians 2:7) "that we might know the things that are freely given to us of God" (1 Corinthians 2:12), understanding what the will of the Lord is. HMM III