First, a quick note on the first sentence of today's reading, let we think that Paul is being overly harsh. He is definitely exasperated with the Galatians, but I'm not sure it's culturally appropriate to follow Paul in this regard:
"The “header” for this section, “You foolish Galatians!” is not intended to make friends, but neither was it perceived as a personal insult and therefore unworthy of an apostolic leader...the latitude for acceptable speech in debate was much greater then than it is today. Such “name calling” and heated rhetoric is found elsewhere in the New Testament, whether on the lips of John the Baptist (Matt. 3:7–10), Jesus (Matt. 23; Luke 24:25), James (James 4:1–12; 4:13–5:6), or Paul elsewhere (2 Cor. 10–13). While we may not imitate Paul in this regard, we can learn about different but acceptable forms of rhetoric.
The term foolish, however, captures Paul’s point: they were illogical in committing themselves to the Pauline message of God’s grace in Christ and then succumbing to the Judaizers’ Moses-gospel. Most doubt that Paul is seriously interested here in the “who,” for he already knows the answer to this question: the Judaizers. Rather, his question is rhetorical in preparation for the following exclamation: “How could you have been fooled2 after learning about the crucifixion of Jesus Christ!” 
Let's take a quick moment today to just emphasize what the point of Galatians is. As we move forward, Paul is going to be speaking a lot about the purpose of the Law and the role it plays. As we saw in the introduction, this book is certainly about the relationship and interaction of Law and Gospel - it is theologically very rich in those areas. THEOLOGICALLY, that is the theme and focus of the book. But I would say that the main force APPLICATIONALLY is what I would call "Gospel forgetfulness". We see it all over this passage. Paul preached the true Gospel of freedom to the people of Galatia, but they "forgot" it, going back to the slavery of the Law. Paul seeks to remind them:
"Did you receive the Spirit by works of the law or by hearing with faith? Are you so foolish? Having begun by the Spirit, are you now being perfected by the flesh? Did you suffer so many things in vain—if indeed it was in vain? Does he who supplies the Spirit to you and works miracles among you do so by works of the law, or by hearing with faith?"
It might seem strange to us at first that people would be so quick to forget the Gospel and go back to regulations and standard they could never possibly live up to. We talked about this yesterday: this was basically the issue that Paul called Peter out on. Peter "forgot" the Gospel life out of fear of man - perhaps that is the same reason the Galatians did, or maybe they were just deceived. Either way, they forgot what it means to be made alive by the Gospel and set free of the burden of the law. As I said, it may seem strange, but only until we look to our own lives.
This is in essence very similar to what we talked about in Romans 7: we seem to seek out law in our lives unless we very purposely live out the Gospel. We set personal standards or follow the standards of our friends or what our culture deems makes a "good" person. For the most part, those who seek God struggle very much with wanting to be "good" by the standards of other people. Paul calls us back from that: the only way we can ever be good is by receiving the righteousness of Jesus. Let us not forget. Let us not seek out any standard other than that by which to be judged "good". It may make us feel great and successful, but ultimately we'll only be just fools as the Galatians were. Remember the Gospel. Live the Gospel. Live in the love of God.