There can be no doubt that in this whole context the Apostle has in mind the great passage in Isaiah lix. where the prophet, in a figure of extreme boldness, describes the Lord as arming Himself to deliver the oppressed faithful, and coming as a Redeemer to Zion. In that passage the Lord puts on righteousness as a breastplate -- that is to say, God, in His manifestation of Himself for the deliverance of His people, comes forth as if arrayed in the glittering armour of righteousness. Paul does not shrink from applying the same metaphor to those who are to be 'imitators of God as beloved children,' and from urging upon them that, in their humble degree and lowly measure, they too are to be clothed in the bright armour of moral rectitude. This righteousness is manifested in character and in conduct, and as the breastplate guards the vital organs from assault, it will keep the heart unwounded.
We must note that Paul here gathers up the whole sum of Christian character and conduct into one word. All can be expressed, however diversified may be the manifestations, by the one sovereign term 'righteousness,' and that is not merely a hasty generalisation, or a too rapid synthesis. As all sin has one root and is genetically one, so all goodness is at bottom one. The germ of sin is living to oneself: the germ of goodness is living to God. Though the degrees of development of either opposite are infinite, and the forms of its expression innumerable, yet the root of each is one.
Paul thinks of righteousness as existent before the Christian soldier puts it on. In this thought we are not merely relying on the metaphor of our text, but bringing it into accord with the whole tone of New Testament teaching, which knows of only one way in which any soul that has been living to self, and therefore to sin, can attain to living to God, and therefore can be righteous. We must receive, if we are ever to possess, the righteousness which is of God, and which becomes ours through Jesus Christ. The righteousness which shines as a fair but unattainable vision before sinful men, has a real existence, and may be theirs. It is not to be self-elaborated, but to be received.
That existent righteousness is to be put on. Other places of Scripture figure it as the robe of righteousness; here it is conceived of as the breastplate, but the idea of assumption is the same. It is to be put on, primarily, by faith. It is given in Christ to simple belief. He that hath faith thereby has the righteousness which is through faith in Christ, for in his faith he has the one formative principle of reliance on God, which will gradually refine character and mould conduct into whatsoever things are lovely and of good report. That righteousness which faith receives is no mere forensic treating of the unjust as just, but whilst it does bring with it pardon and oblivion from past transgressions, it makes a man in the depths of his being righteous, however slowly it may afterwards transform his conduct. The faith which is a departure from all reliance on works of righteousness which we have done, and is a single-eyed reliance on the work of Jesus Christ, opens the heart in which it is planted to all the influences of that life which was in Jesus, that from Him it may be in us. If Christ be in us (and if He is not, we are none of His), 'the spirit is life because of righteousness,' however the body may still be 'dead because of sin.'
But the putting on of the breastplate requires effort as well as faith, and effort will be vigorous in the measure in which faith is vivid, but it should follow, not precede or supplant, faith. There is no more hopeless and weary advice than would be the exhortation of our text if it stood alone. It is a counsel of despair to tell a man to put on that breastplate, and to leave him in doubt where he is to find it, or whether he has to hammer it together by his own efforts before he can put it on. There is no more unprofitable expenditure of breath than the cry to men, Be good! Be good! Moral teaching without Gospel preaching is little better than a waste of breath.
This injunction is continuously imperative upon all Christian soldiers. They are on the march through the enemy's country, and can never safely lay aside their armour. After all successes, and no less after all failures, we have still to arm ourselves for the fight, and it is to be remembered that the righteousness of which Paul speaks differs from common earthly moralities only as including and transcending them all. It is, alas, too true that Christian righteousness has been by Christians set forth as something fantastic and unreal, remote from ordinary life, and far too heavenly-minded to care for common virtues. Let us never forget that Jesus Himself has warned us, that except our righteousness exceed the righteousness of the Scribes and Pharisees, we shall in no wise enter the Kingdom of Heaven. The greater orbit encloses the lesser within itself.
The breastplate of righteousness is our defence against evil. The opposition to temptation is best carried on by the positive cultivation of good. A habit of righteous conduct is itself a defence against temptation. Untilled fields bear abundant weeds. The used tool does not rust, nor the running water gather scum. The robe of righteousness will guard the heart as effectually as a coat of mail. The positive employment with good weakens temptation, and arms us against evil. But so long as we are here our righteousness must be militant, and we must be content to live ever armed to meet the enemy which is always hanging round us, and watching for an opportunity to strike. The time will come when we shall put off the breastplate and put on the fine linen 'clean and white,' which is the heavenly and final form of the righteousness of Saints.