John Calvin Commentary Ephesians 5:17

"Wherefore be ye not foolish, but understand what the will of the Lord is." — Ephesians 5:17 (ASV)

Therefore do not be unwise. He whose delight is in the law of the Lord, and who meditates in it day and night (Psalms 1:2), will triumph over every obstacle that Satan can oppose to his progress. How does it happen that some wander, others fall, others strike against a rock, and others go away? It is because we allow ourselves to be gradually blinded by Satan and lose sight of the will of God, which we ought constantly to remember. And observe that Paul defines wisdom as understanding what the will of the Lord is.

How shall a young man, says David, direct his way? By attending to your word, O Lord (Psalms 119:9).

He speaks of youths, but it is the same wisdom that belongs to old men.