From Dr. Luther's 1535 Lectures on Galatians:
"‘You were called to freedom, brethren; only do not use your freedom as an opportunity for the flesh, but through love be servants of one another’ (Gal. 5:13). It is as though Paul were saying: ‘Now you have obtained freedom through Christ. That is, you are far above all laws, both in your own conscience and in the sight of God; you are blessed and saved; Christ is your life. Therefore even though the Law, sin, and death may frighten you, they can neither harm you nor cause you to despair. This is your brilliant and inestimable freedom. Now it is up to you to be diligently on your guard not to use your freedom as an opportunity for the flesh.’
"This evil is very widespread, and it is the worst of all the evils that Satan arouses against the teaching of faith: that in many people he soon transforms the freedom for which Christ has set us free into an opportunity for the flesh. Jude complains of this same thing in his Epistle (v. 4): ‘Admission has been secretly gained by some ungodly persons who pervert the grace of our God into licentiousness.’ For the flesh simply does not understand the teaching of grace, namely, that we are not justified by works but by faith alone, and that the Law has no jurisdiction over us. Therefore when the flesh hears this teaching, it transforms it into licentiousness and immediately draws the inference: ‘If we are without the Law, then let us live as we please. Let us not do good, let us not give to the needy; much less do we have to endure anything evil. For there is no Law to compel or bind us.’
"Thus there is a danger on both sides, although the one is more tolerable than the other. If grace or faith is not preached, no one is saved; for faith alone justifies and saves. On the other hand, if faith is preached, as it must be, the majority of men understand the teaching about faith in a fleshly way and transform the freedom of the Spirit into the freedom of the flesh. This can be discerned today in all classes of society, both high and low. They all boast of being evangelicals and boast of Christian freedom. Meanwhile, however, they give in to their desires and turn to greed, sexual desire, pride, envy, etc. No one performs his duty faithfully; no one serves another by love. This misbehavior often makes me so impatient that I would want such ‘swine that trample pearls underfoot’ (Matt. 7:6) still to be under the tyranny of the pope. For it is impossible for this people of Gomorrah to be ruled by the Gospel of peace.
"What is more, we ourselves, who teach the Word, do not perform our own duty with as much care and zeal here in the light of the truth as we used to in the darkness of ignorance. The more certain we are about the freedom granted to us by Christ, the more unresponsive and slothful we are in presenting the Word, praying, doing good works, enduring evil, and the like. And if Satan were not troubling us inwardly with spiritual trials and outwardly with persecution by our enemies and with the contempt and ingratitude of our own followers, we would become utterly smug, lazy, and useless for anything good; thus in time we would lose the knowledge of Christ and faith in Him, would forsake the ministry of the Word, and would look for some more comfortable way of life, more suitable to our flesh. This is what many of our followers are beginning to do, motivated by the fact that those who labor in the Word not only do not get their support from this but are even treated shamefully by those whom their preaching of the Gospel has set free from the miserable slavery of the pope. Forsaking the poor and offensive figure of Christ, they involve themselves in the business of this present life; and they serve, not Christ but their own appetites (Rom. 16:18), with results that they will experience in due time.
"We know that the devil lies in wait especially for us who have the Word — he already holds the others captive to his will — and that he is intent upon taking the freedom of the Spirit away from us or at least making us change it into license. Therefore we teach and exhort our followers with great care and diligence, on the basis of Paul’s example, not to think that this freedom of the Spirit, achieved by the death of Christ, was given to them as an opportunity for the flesh or, as Peter says, ‘to use as a pretext for evil’ (1 Peter 2:16), but for them to be servants of one another through love.
"Therefore, the Apostle Paul imposes an obligation on Christians through this law about mutual love in order to keep them from abusing their freedom. Therefore the godly should remember that for the sake of Christ they are free in their conscience before God from the curse of the Law, from sin, and from death, but that according to the body they are bound; here each must serve the other through love, in accordance with this commandment of Paul.
"Therefore let everyone strive to do his duty in his calling and to help his neighbor in whatever way he can. This is what Paul requires of us with the words ‘through love be servants of one another,’ which do not permit the saints to run free according to the flesh but subject them to an obligation.
"Of course, it is impossible to teach or persuade unspiritual people of this teaching about the love that is to be mutually observed among us. Christians comply with it voluntarily. But when the others hear this freedom proclaimed, they immediately draw the inference: ‘If I am free, then I have the right to do whatever I please. This thing belongs to me; why should I not sell it for as much as I can? Again, if we do not obtain salvation on account of good works, why should we give anything to the poor?’ In their great smugness such people shrug off this yoke and obligation of the flesh, and they transform the freedom of the Spirit into the license and lust of the flesh. Although they will not believe us but will make fun of us, we make this sure announcement to these smug despisers: If they use their bodies and their powers for their own lusts — as they are certainly doing when they refuse to help the poor and to share, but defraud their brethren in business and acquire things by fair means or foul — then they are not free, as they loudly claim to be, but have lost both Christ and freedom, and are slaves of the devil, so that now, under the title of ‘Christian freedom,’ their state is seven times as bad as it used to be under the tyranny of the pope. For when the devil who has been cast out of them returns to them, he brings with him seven spirits more evil than himself. Therefore their last state becomes worse than the first (Matt. 12:43–45).
"We for our part have the divine command to preach the Gospel, which announces to all men, if only they believe, the free gift of freedom from the Law, from sin, from death, and from the wrath of God, for the sake of Christ. We have neither the intention nor the authority to conceal this freedom or to obscure and cancel it once it has been made public through the Gospel; for Christ has granted it to us and has achieved it by His death. Nor are we able to compel those swine, who are rushing headlong into the license of the flesh, to be servants of others with their bodies and their possessions. Therefore we do what we can. That is, we diligently admonish them that this is what they should do. If we do not accomplish anything with these warnings of ours, we commit the matter to God, to whom it belongs anyway. In His own time He will inflict just punishment on them. Meanwhile, however, we are comforted by the fact that our labor and our diligence are not in vain among the godly, many of whom have undoubtedly been rescued by our ministry from the slavery of the devil and have been transferred to the freedom of the Spirit. These few — who acknowledge the glory of this freedom, who at the same time are ready to be the servants of others through love, and who know that according to the flesh they are debtors to the brethren — give us a happiness that is greater than the sadness that can be caused by the infinite number of those who abuse this freedom.
"Paul speaks in clear and precise terms when he says: ‘You were called to freedom.’ To prevent anyone from imagining that he means the freedom of the flesh, he explains himself and says what kind of freedom he has in mind: ‘Only do not use your freedom as an opportunity for the flesh, but through love be servants of one another.’ Therefore every Christian should know that in his conscience he has been established by Christ as a lord over the Law, sin, and death, and that they do not have jurisdiction over him. On the other hand, he should know also that this external obligation has been imposed on his body, that through love he should serve his neighbor. Those who understand Christian freedom differently are enjoying the advantages of the Gospel to their own destruction and are worse idolaters under the name ‘Christian’ than they used to be under the pope." (Luther’s Works, Volume 27, CPH 1963)