By “evil day” he means the present life, and calls it too “this present evil world”, from the evils which are done in it. It is as much as to say, Always be armed. And again, “having done all,” says he; that is, both passions, and vile lusts, and all things else that trouble us. He speaks not merely of doing the deed, but of completing it, so as not only to slay, but to stand also after we have slain. For many who have gained this victory, have fallen again. “Having done,” says he, “all”; not having done one, but not the other. For even after the victory, we must stand. An enemy may be struck, but things that are struck revive again if we do not stand. But if after having fallen they rise up again, so long as we stand, they are fallen. So long as we waver not, the adversary rises not again.
“Let us put on the whole armor of God.” Do you see how he banishes all fear? For if it be possible “to do all, and to stand,” his describing in detail the power of the enemy does not create cowardice and fear, but it shakes off indolence. “That ye may be able,” he says, “to withstand in the evil day.” And he further gives them encouragement too from the time; the time, he seems to say, is short; so that you must needs stand; faint not when the slaughter is achieved.
Moral. If then it is a warfare, if such are the forces arrayed against us, if “the principalities” are incorporeal, if they are “rulers of the world,” if they are “the spiritual hosts of wickedness,” how, tell me, can you live in self-indulgence? How can you be dissolute? How if we are unarmed, shall we be able to overcome? These words let every one repeat to himself every day, whenever he is under the influence of anger, or of lust, whenever he is aiming, and all to no profit, after this languid life. Let him hearken to the blessed Paul, saying to him, “Our wrestling is not against flesh and blood, but against the principalities, against the powers.” A harder warfare this than that which is matter of sense, a fiercer conflict. Think how long time this enemy is wrestling, for what it is that he is fighting, and be more guarded than ever. “Nay,” a man will say, “but as he is the devil, he ought to have been removed out of the way, and then all had been saved.” These are the pretenses to which some of your indolent ones in self-defense give utterance. When you ought to be thankful, O man, that, if you have a mind, you have the victory over such a foe, you are on the contrary even discontented, and givest utterance to the words of some sluggish and sleepy soldier. You know the points of attack, if you choose. Reconnoiter on all sides, fortify yourself. Not against the devil alone is the conflict, but also against his powers. How then, you may say, are we to wrestle with the darkness? By becoming light. How with the “spiritual hosts of wickedness”? By becoming good. For wickedness is contrary to good, and light drives away darkness. But if we ourselves too be darkness, we shall inevitably be taken captive. How then shall we overcome them? If, what they are by nature, that we become by choice, free from flesh and blood, thus shall we vanquish them. For once it was probable that the disciples would have many persecutors, “imagine not,” he would say, “that it is they who war with you. They that really war with you, are the spirits that work in them. Against them is our conflict.” Two things he provides for by these considerations; he renders them in themselves more courageous and he lets loose their wrath against those who war against them. And wherefore is our conflict against these? Since we have also an invincible ally, the grace of the Spirit. We have been taught an art, such as shall enable us to wrestle not against men, but against spirits. Nay, if we have a mind, neither shall we wrestle at all; for it is because we choose it, that there is a struggle, since so great is the power of Him that dwells in us, as that He said, “Behold, I have given you authority to tread upon serpents and scorpions, and over all the power of the enemy.” All power has He given us, both of wrestling and of not wrestling. It is because we are slothful, that we have to wrestle with them; for that Paul wrestled not, hear what he says himself, “Who shall separate us from the love of Christ? Shall tribulation, or anguish, or persecution, or famine, or nakedness, or peril, or sword?” And again hear his words, “God shall bruise Satan under your feet shortly.” For he had him under his subjection; whence also he said, “I charge you in the name of Jesus Christ to come out of her.” And this is not the language of one wrestling; for he that wrestles has not yet conquered, and he that has conquered no longer wrestles; he has subdued, has taken his captive. And so Peter again wrestled not with the devil, but he did that which was better than wrestling. In the case of the faithful, the obedient, the catechumens, they prevailed over him to vast advantage and over his powers. Hence too was it that the blessed Paul said, “For we are not ignorant of his devices”, which was the way moreover in which he especially overcame him; and again hear his words, “And no marvel— if his ministers also fashion themselves as ministers of righteousness.” So well knew he every part of the conflict, and nothing escaped him. Again, “For the mystery of lawlessness,” says he, “does already work.”
Source: Homilies on Ephesians (New Advent)