In the second half of Galatians 1 (and halfway into chapter 2), Paul recounts with extensive detail how he learned (and did not learn) the gospel he was called to preach. He does this because the validity of his apostolic ministry was being called into question by other church leaders, and especially by those who have come to be known as the "Judaizers." As Luther writes, "Paul was forced to speak of his conversion to combat the slanderous contention of the false apostles to the effect that this apostleship was inferior to that of the other apostles."
And Luther himself takes the opportunity afforded by the writing of his commentary to defend his own ministry against the accusations of his critics:
The arguments which the false apostles advanced impress people to this day. "Who are you to dissent from the fathers and the entire Church, and to bring a contradictory doctrine? Are you wiser than so many holy men, wiser than the whole Church?" When Satan, abetted by our own reason, advances these arguments against us, we lose heart, unless we keep on saying to ourselves: "I don't care if Cyprian, Ambrose, Augustine, Peter, Paul, John, or an angel from heaven, teaches so and so. I know that I teach the truth of God in Christ Jesus."
The letter to Galatians is not the only time Paul defends his character and ministry (he does it throughout the book of 2 Corinthians as well). And the commentary on Galatians is not the only time Luther did it either (the Diet of Worms being one prominent example, of course).
Perhaps you have wondered, like I have...When is it right and wise to defend ourselves, and when should we not? (1 Corinthians 6:7 even says, "Why not rather be wronged?") That's always a tough question, but maybe Paul's and Luther's words can provide some guidance for us...
Defend yourself when the truth of God's Word, and the honor of God himself, is at stake.
This is obviously Paul's concern in Galatians 1, where at the beginning of the passage he says that his gospel was received through "a revelation of Jesus Christ" (v. 12), and at the end he says "they were glorifying God because of me" (v. 24). Luther also has the same concerns:
The article of justification is fragile. Not in itself, of course, but in us. I know how quickly a person can forfeit the joy of the Gospel. I know in what slippery places even those stand who seem to have a good footing in the matters of faith. In the midst of the conflict when we should be consoling ourselves with the Gospel, the Law rears up and begins to rage all over our conscience. I say the Gospel is frail because we are frail.
What makes matters worse is that one-half of ourselves, our own reason, stands against us. The flesh resists the spirit, or as Paul puts it, "The flesh lusteth against the Spirit." Therefore we teach that to know Christ and to believe in Him is no achievement of man, but the gift of God. God alone can create and preserve faith in us. God creates faith in us through the Word. He increases, strengthens and confirms faith in us through His word. Hence the best service that anybody can render God is diligently to hear and read God's Word. On the other hand, nothing is more perilous than to be weary of the Word of God. Thinking he knows enough, a person begins little by little to despise the Word until he has lost Christ and the Gospel altogether.
Let every believer carefully learn the Gospel. Let him continue in humble prayer. We are molested not by puny foes, but by mighty ones, foes who never grow tired of warring against us. These, our enemies, are many: Our own flesh, the world, the Law, sin, death, the wrath and judgment of God, and the devil himself.
Defend yourself if your ability to help others is being hindered.
Paul and Luther weren't worried about the effects that misunderstanding and slander would have on their personal pride, their ability to make money, or their reputation for it's own sake. Rather, for the welfare of others they took the time and energy to respond to public criticism.
This can be seen in the whole tone and context of Paul's words in this passage, and also in 2 Corinthians, where he sometimes even seems apologetic that he has to defend himself when he'd rather not. Early in that book, he says in Chapter 4 verse 15, "For all things are for your sakes, so that the grace which is spreading to more and more people may cause the giving of thanks to abound to the glory of God."
Luther likewise says, "Why does Paul harp on this seemingly unimportant fact [that he didn't learn anything from the other apostles]? To convince the churches of Galatia that his Gospel was the true Word of Christ which he learned from Christ Himself and from no man. Paul was forced to affirm and re-affirm this fact. His usefulness to all the churches that had used him as their pastor and teacher was at stake."
In many situations, it's not necessarily right or wrong to defend yourself...only you and God can decide whether you should or not. But if your motivation is not primarily love for God and others, and it's more about you, "why not rather be wronged?" It's an opportunity to follow in the steps of Jesus, "who committed no sin, nor was any deceit found in his mouth; and while being reviled, he did not revile in return; while suffering, he uttered no threats, but kept entrusting Himself to God who judges righteously." (1 Peter 2:22-23).