Wherefore be ye not unwise. He whose "delight is in the law of the Lord, and who meditates in it day and night," (Psalm 1:2,) will triumph over every obstacle which Satan can oppose to his progress. Whence comes it that some wander, others fall, others strike against a rock, others go away, -- but 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 to be, understanding what the will of the Lord is "How shall a young man," says David, "direct his way? By attending to thy word, O Lord." (Psalm 119:9.) He speaks of youths, but it is the same wisdom which belongs to old men.

Be ye not unwise - Be not fools in the employment of your time, and in your manner of life. Show true wisdom by endeavoring to understand what the will of the Lord is, and then doing it.

Wherefore be ye not unwise - Μη γινεσθε αφρονες· Do not become madmen. Here is a most evident allusion to the orgies of Bacchus, in which his votaries acted like madmen; running about, tossing their heads from shoulder to shoulder, appearing to be in every sense completely frantic. See the whole of the passage in Livy, to which I have referred on Ephesians 5:12.
But understanding what the will of the Lord is - It is the will of God that ye should be sober, chaste, holy, and pure. Get a thorough understanding of this; acquaint yourselves with God's will, that ye may know how to glorify him.

Wherefore be ye not unwise,.... No one would be thought to be unwise, but such are, who do not redeem time, and are ignorant of the will of the Lord; believers should not act the unwise part, neither in their talk, nor in their walk and conversation, nor in their use of time:
but understanding what the will of the Lord is; or "of God", as read the Alexandrian copy, the Vulgate Latin, Syriac, and Ethiopic versions: there is the secret will of God, which is the rule of all his proceedings; and is unknown to men, till facts make it appear; this is always fulfilled, and sometimes by persons who have no regard to his revealed will; to this the wills of the people of God should be always resigned: and there is his revealed will, which lies partly in the Gospel; which declares it to be his will, that Christ should work out the salvation of his people, which is what he came to do; that whoever believes in him shall be saved; that all that are redeemed shall be sanctified; and that they shall persevere to the end, and be glorified; and partly in the law, in the precepts and commands of it, which contain the good, perfect, and acceptable will of God: and the understanding of it is not a mere speculative knowledge of it, but a practical one; when a man not only knows, but does the will of God, and his heart and actions agree with it; and this is to be done in faith, in virtue of grace and strength received, with a view to the glory of God, having no dependence on what is done; and to the right understanding of it, so as to act according to it, as should be, the word of God, and the illuminations, instructions, and grace of the Spirit, are necessary: the Alexandrian copy, Syriac, Arabic, and Ethiopic versions, read the words as an exhortation, "understand ye the will of God".

Wherefore--seeing that ye need to walk so circumspectly, choosing and using the right opportunity of good.
unwise--a different Greek word from that in Ephesians 5:15. Translate, "foolish," or "senseless."
understanding--not merely knowing as a matter of fact (Luke 12:47), but knowing with understanding.
the will of the Lord--as to how each opportunity is to be used. The Lord's will, ultimately, is our "sanctification" (1-Thessalonians 4:3); and that "in every thing," meantime, we should "give thanks" (1-Thessalonians 5:18; compare above, Ephesians 5:10).

What the will of the Lord is - In every time, place, and circumstance.