Paul appears to be extremely passionate about the false doctrines associated with legalism. The specific issue with the Galatians was the necessity of circumcision for the gentile converts. Paul is so adamant and so 
passionate that in Galatians 5:12 he explodes with, “I wish those who unsettle you would emasculate themselves!” According to Maxie Dunnam, “Galatia was near Phrygia where people worshiped Cybele. The priests and sometimes other devout worshipers, in frenzied devotion, mutilated themselves by castration. Cybele priests were eunuchs.” There are other passages in the Bible that speak of religious “Eunuchs” as well. Philip shared the Gospel with one from Ethiopia in the book of Acts. Dunnam goes on, “The Galatians who knew about these priests could not miss his inference. If salvation depended on the merit of a physical operation, circumcision, why not go all the way and castrate yourselves like the heathen priests? Wouldn’t this more drastic rite give greater assurance than the Jewish custom?”
This is a rather crude and course illustration but it would have been very dramatically real for the Galatians, some of whom had been delivered from pagan cults who may have practiced such things. Being the kind of person who often says things he’s sorry for, I can’t help but try to feel what Paul felt after having said that. I’m not suggesting that this phrase in Paul’s letter is any less inspired than the rest, it’s just that in the next verse, Galatians 5:13, Paul’s voice seems to get calm and he begins his phrase with “brothers.” He writes, “Brothers…you were called to freedom. Only do not use your freedom as an opportunity for the flesh, but through love serve one another.
I know that some people use this radical liberty as occasions to sin. Paul dealt with that side of the issue as well. Some are afraid to preach God’s unadulterated grace because of this danger. But there is no real danger for those who are indwelt by the Spirit. The interesting thing to me is that whether you believe in grace alone, or grace plus some work, you will still sin at times. That’s the nature of our depravity. During our failures it’s obvious we’ve stopped responding to the Spirit’s promptings and have taken matters into our own hands. Richison sums it up well, “The Christian life is freedom from sin, not the freedom to sin. If we use grace as an excuse to sin, we do not understand the essence of freedom from sin through grace. God never issues a license to commit sin.”