From the Word
11 Besides this you know what hour it is, how it is full time now for you to wake from sleep. For salvation is nearer to us now than when we first believed; 12 the night is far gone, the day is at hand. Let us then cast off the works of darkness and put on the armor of light; 13 let us conduct ourselves becomingly as in the day, not in reveling and drunkenness, not in debauchery and licentiousness, not in quarreling and jealousy. 14 But put on the Lord Jesus Christ, and make no provision for the flesh, to gratify its desires.
Romans 13:11–14, RSV
We are not profited by the shining of the sun, and the day it produces, if our eyes fail to perceive its light. Similarly, though the gospel is revealed and Christ is proclaimed to the world, none are enlightened but those who receive it, who have risen from sleep through the agency of the light of faith. They who sleep are not affected by the sun and the day; they receive no light and receive as little as if there were neither sun nor day. It is to our day that Paul refers when he says: “Knowing the season, that already it is time for you to wake out of sleep.” In the light of our spiritual knowledge we are to rise from sleep and lay aside the works of darkness. Paul is not addressing unbelievers. He tells the Romans they know the time is at hand, that the night is past and the dawn appears.
But why this passage to believers? Because no one ever gets to the point of knowledge where it is not necessary to admonish him, continually to urge him to new reflections upon what he already knows; for there is danger of his untiring enemies — the devil, the world and the flesh — wearying him and causing him to become negligent, and ultimately lulling him to sleep. There should, therefore, be continuous exhorting to vigilance and activity. Hence the Holy Spirit is called the Comforter or Helper, who incites and urges to good.
Not the works of darkness Paul calls “armor,” but the works of light. Why “armor” rather than “works?” Doubtless to teach that only at the cost of conflicts, pain, labor and danger will the truly watchful and godly life be maintained. But it is no easy thing to stand always in battle array during the whole life. Good trumpets and bugles are necessary, preaching and exhortation of a sort to enable us valiantly to maintain our position in battle. Good works are armor. Let not the works of darkness get such control of you as to render your members weapons of unrighteousness. The word “light” here carries the thought of faith. The “armor of light” is simply the works of faith. “Darkness” is unbelief; it reigns in the absence of the gospel and of Christ through the instrumentality of the doctrines of men, instigated by the devil.
Martin Luther and John Sander, Devotional Readings from Luther’s Works for Every Day of the Year (Rock Island, IL: Augustana Book Concern, 1915), 446–447.