Ephesians 6:13

That is, in the present age, because he calls it an evil day from the evil deeds committed in it. He also encourages by pointing out that the time of struggle is brief. For, by calling it a day, he indicates its brevity. Having overcome …

In the evil day - The war is perpetual; but the fight is one day less, another more, violent. The evil day is either at the approach of death, or in life; may be longer or shorter and admits of numberless varieties.

Wherefore take unto you the whole armour of God,....

Satan is said to stand up against us, 1 Chron. xxi. If he stand up against us, we must stand against him; set up, and keep up, an interest in opposition to the devil. Satan is the wicked one, and his kingdom is the kingdom of sin: to stand against Satan is to strive against sin.

In the evil day - The day of temptation; the day when you are violently assaulted. And having done all, to stand - Margin, “or overcome.” The Greek word means, to work out, effect, or produce; and then to work up, to make an end of, to vanquish. Robinson, Lexicon. The idea seems to be, that they were to overcome or vanquish all their foes, and thus to stand firm. The whole language here is taken from war; and the idea is, that every foe was to be subdued - no matter how numerous or formidable they might be.

We must have our armor always on, to be ready against the evil day which may come at any moment, the war being perpetual (Ps 41:1, Margin). done all--rather, "accomplished all things," namely, necessary to the fight, and becoming a good soldier.

Wherefore - Because ye have such enemies to contend with, take unto you - assume, as provided and prepared for you, the whole armor of God; which armor if you put on and use, you shall be both invulnerable and immortal. The ancient heroes are fabled to have had armor sent to them by the gods; and even the great armor-maker, Vulcan, was reputed to be a god himself. This was fable: What Paul speaks of is reality. See before on Eph 6:11 (note). That ye may be able to withstand - That ye may not only stand fast in the liberty wherewith Christ hath made you free, but also discomfit all your spiritual foes; and continuing in your ranks, maintain your ground against them, never putting off your armor, but standing always ready prepared to repel any new attack. And having done all, to stand - Και ἁπαντα κατεργασαμενοι στηναι· rather, And having conquered all, stand: this is a military phrase, and is repeatedly used in this sense by the best Greek writers. So Dionys. Hal. Ant., lib. vi., page 400: Και παντα πολεμια εν ολιγῳ κατεργασαμενοι χρονῳ· “Having in a short time discomfited all our enemies, we returned with numerous captives and much spoil.” See many examples in Kypke. By evil day we may understand any time of trouble, affliction, and sore temptation. As there is here allusion to some of the most important parts of the Grecian armor, I shall give a short account of the whole. It consisted properly of two sorts: 1. Defensive armor, or that which protected themselves. 2. Offensive armor, or that by which they injured their enemies. The apostle refers to both. I. Defensive Armor Περικεφαλαια, the Helmet; this was the armor for the head, and was of various forms, and embossed with a great variety of figures. Connected with the helmet was the crest or ridge on the top of the helmet, adorned with several emblematic figures; some for ornament, some to strike terror. […]

Notice that Paul assumes the evil day is coming. I do not believe this refers to the Great Tribulation. It means that every Christian is going to go through his own particular evil day.