"‘Do not submit again to a yoke of slavery’ (Galatians 5:1). Paul has been speaking very seriously about grace and Christian freedom, and has urged the Galatians in many words to continue in these. He commands them to stand, because it is very easy to lose all this either by carelessness and smugness or by a relapse from grace and faith into the Law and works. But because to reason this does not seem to be dangerous, since reason vastly prefers the righteousness of the law to the righteousness of faith, therefore he denounces the Law of God with great indignation; contemptuously and scornfully he calls it a ‘yoke,’ in fact, a ‘yoke of slavery.’ That is how Peter spoke in Acts (15:10): ‘Why do you make trial of God by imposing a yoke?’
"In this way Paul turns the tables completely. For the false apostles minimized the importance of the promise and magnified the Law and its works in the following way: ‘If you want to be set free from sin and death, and to obtain righteousness and life, keep the Law; be circumcised; observe days, months, seasons, and years; perform sacrifices. Then this obedience to the Law will justify and save you.’ Paul says the exact opposite: ‘Those who teach the Law in this way,’ he says, ‘do not set consciences free; they ensnare them. They ensnare them in a yoke, indeed in a yoke of slavery.’
"Therefore Paul speaks with complete contempt and in an exceedingly reproachful manner about the Law when he calls it a snare of the harshest slavery and of a servile yoke. He does not do this without reason. The wicked notion that the Law justifies clings to the reason very stubbornly, and the whole human race is finally so entangled and conquered by it that it can be rescued only with the utmost difficulty. Here Paul seems to be comparing those who seek righteousness through the Law to oxen that have been subjected to a yoke. Just as oxen that bear the yoke with great effort get nothing out of it but their food and are slaughtered when they are no longer fit to bear the yoke, so those who seek righteousness in the Law are captive and are oppressed with a yoke of slavery, that is, with the Law; and when finally, after great effort and sorrow, they have worn themselves out with the works of the Law, all the reward they get is that they are miserable slaves forever. Slaves of what? Of sin, death, the wrath of God, the devil, the flesh, the world, and all creatures. Therefore no slavery is greater or more severe than the slavery of the Law. Hence it is not without reason that Paul calls it ‘a yoke of slavery’; for, as we have often said, the Law only demonstrates and increases sin, accuses, terrifies, condemns, works wrath, and finally brings consciences to the point of despair — which is the most wretched and the harshest slavery (Romans 3, 4, 7).
"This is why Paul uses such passionate words. He would dearly love to stir and persuade them not to let themselves be influenced by the false apostles and not to let these men ensnare them once more in the yoke of slavery. It is as though he were saying: ‘The issue here is no trifle or mere nothing; it is an issue between either endless, eternal freedom or slavery.’ For just as the freedom from the wrath of God and from every evil is not political freedom or a freedom of the flesh but an eternal freedom, so the slavery of sin, death, and the devil, which oppresses those who seek to be justified and saved through the Law, is not a physical slavery, which lasts for a while, but a perpetual slavery. For self-righteous people of this kind, who take everything very seriously — and they are the ones whom Paul is discussing — are never serene and peaceful. In this life they are always in doubt about the will of God and are afraid of death and of the wrath and judgment of God; and after this life they will suffer eternal destruction as punishment for their unbelief.
"Therefore the workers of the Law are very rightly called ‘martyrs of the devil,’ if I may use the common expression. They earn hell by greater toil and trouble than that by which the martyrs of Christ earn heaven. They are worn down by a double contrition: while they are in this life, performing many great works, they torture themselves miserably without reason; and when they die, they receive eternal damnation and punishment as their reward. Thus they are most miserable martyrs both in the present life and in the future life, and their slavery is eternal. It is not so with believers, who have troubles only in the present life. Therefore we must stand fast in the freedom Christ has acquired for us by His death, and we must be diligently on our guard not to be ensnared once more in a yoke of slavery. This is what is happening today to the fanatical spirits: falling away from faith and freedom, they have a self-imposed temporal slavery in this life, and in the life to come they will be oppressed by an eternal slavery. The papists do not listen to the Gospel; they persecute it. But even though these men use the freedom of the Gospel — for many of them are Epicureans — they are really slaves of the devil, who holds them captive at his pleasure. Therefore the eternal slavery of hell awaits them." (Luther’s Works, Volume 27, CPH 1963; alt.)