From Luther’s Commentary on Paul’s letter to the Galatians.
Galatians 3:2: “Did you receive the Spirit by observing the law, or by believing what you heard?”
It seems to reason unlikely that the Holy Spirit is received only by believing what we hear and that there is nothing else required of us but rather we must set aside all our works and give ourselves only to hearing the Gospel. The human heart does not understand or believe that such a great treasure—namely the Holy Spirit—is given only by believing what we hear. We reason that forgiveness of sins, deliverance from death, the gifts of the Holy Spirit, righteousness, and everlasting life are great things, and therefore we must do something that is great if we want to obtain these inestimable benefits. The devil approves of this idea and makes it grow in our hearts. So when our reason hears that we can do nothing to obtain the forgiveness of sins but must only hear the Word of God, it immediately cries out, “Rubbish! You are making the forgiveness of sins too unimportant a matter.” The inestimable greatness of the gift keeps us from believing it; because this incomparable treasure is offered freely, it is despised.
But we must learn that forgiveness of sins, Christ, and the Holy Spirit are given to us freely, simply by believing what we hear preached, despite our horrible sins. We must not think about the greatness of what is given, nor about how unworthy we are of it (for if we did that, we would be terrified); rather, we must think how it pleases God to give us this unutterable gift freely (Luke 12:32). If he wants to give it, I must not consider my own sin and unworthiness but only his fatherly goodwill toward me; I must receive this great gift with joy and gladness and be thankful.
Here again, foolish reason is offended and says, “This makes grace look contemptible, and people will be complacent, idle, and dissolute, so that they do no good at all. So it is not a good idea to preach this doctrine; it cannot be true. People must be urged to labor and exercise themselves for righteousness, and then they will obtain this gift.” This is the very same thing that Pelagius said in the past. But listen to what Paul says in this passage: You have received the Holy Spirit not by your own labor, but by believing what you heard. Martha was busy and could hardly stand her sister Mary’s listening to Jesus; but listen to what Christ says: “Martha, Martha, you are worried and upset about many things, but only one thing is needed. Mary has chosen what is better, and it will not be taken away from her” (Luke 10: 41-42). We become Christians, therefore, not by working but by listening; therefore, those who want to exercise themselves for righteousness must first exercise themselves in listening to the Gospel. When they have heard and received the Gospel, they should give thanks to God gladly, and then they can exercise themselves in those good works that are commanded in the law, so that the law and works may follow belief in what is heard. Thus they may quietly walk in the light—that is, follow Christ—and boldly choose and do works, not hypocritically, but doing things that they know please God and are commanded by him.