Galatians 3:2 “I would like to learn just one thing from you: Did you receive the Spirit by observing the law, or by believing what you heard?”
“. . . learn just one thing” I think Paul’s use of this first phrase is more than just a debating technique. In the rest of Chapter 3 and on into the following chapters, Paul will argue his point from a theological perspective, using examples from Old Testament scripture, specifically the law of Moses, and in particular the life of Abraham. But first, he appeals to matters of personal experience. The Galatians were certainly more than just marginal Christians. They had “received the Spirit.” They had been baptized in the Holy Spirit! I presume this meant all that went along with the outpouring of the Holy Spirit – both the charismatic manifestations and the inner transformation/life change discussed in more detail in Galatians 5. Paul is therefore presenting the totality of his argument, at least at the front end, as an appeal to personal experience in the Holy Spirit.
"I would like to learn just one thing from you" -- he's saying, rhetorically, “tell me one thing – “ as if to say, “is what you experienced a lie? Were those manifestations of the Holy Spirit's power NOT real?” Now, many evangelical apologists will argue against manifestational gifts (in Greek, "charismata"), using theology. They will argue that we can’t use experience, or what we feel to justify miraculous manifestations. But Paul’s statement here in verse 2 is not an argument to justify the concept of “trust your emotions,” or at least to do so at the expense of God’s Word. Paul’s admonitions regarding the proper use of the charismatic gifts is laid out in detail in his first letter to the Corinthians. No, I believe what Paul is saying here is simply that the free gift of the Holy Spirit, the power of the Spirit that transformed the Galatians, and the authentic experiences the Galatians had already had in Christ were enough proof in and of themselves that work-based theology is a lie. (For if our own experience shows us that God is a God who bestows his forgiveness, gifts, and life changing power in our lives when we don’t deserve it, and those experiences are authentic and objectively quantifiable, then how can we then be expected to perform up to some standard in order to “earn” them?). Paul expounds on this further in the next few verses. But the living example of the Holy Spirit at work in our own lives, when we are sinners, should be enough proof on its own to close the book on this argument.