Paul reprimands the Galatians in Chapter 3 because of the influence of the so called "Judaizers," who insisted that Gentile Christians must follow the Jewish law. Because the early Christians were all Jewish and had been circumcised and compelled to follow the law, they believed that the Galatians should do the same. Paul's argument is that one is freed from the law when one becomes a Christian. The following quote from the third chapter illustrates the point:
Did you receive the Spirit by the works of the law, or by believing what you heard? Are you so foolish? After beginning by means of the Spirit, are you now trying to finish by means of the flesh? Have you experienced so much in vain—if it really was in vain? So again I ask, does God give you his Spirit and work miracles among you by the works of the law, or by your believing what you heard?
In other words, they have heard the word of Christ preached to them; but some still stubbornly held to the law; including the law of circumcision and the dietary laws. Since Christ had redeemed Christians by his death on the cross, the purpose of the law was fulfilled, and there was no longer any need to be bound by it. To insist on strict observance of the law was to Paul, folly. For that reason, he speaks rather harshly to the members of the Galatian Church.