Gal 5:12 As for those agitators, I wish they would go the whole way and emasculate themselves!
While Paul's expressing his anger at those false teachers, he's also being humorous here. Circumcision is the cutting off of the foreskin of the penis. (You probably never guessed you run into the word "penis" in a Bible study guide!) Well what Paul was saying was that if they thought that circumcision was such a big deal, if indeed God has made salvation contingent upon cutting off a piece of skin, then why don't they just cut their penis off altogether!
This is not simply an insult, it's part of his persuasive argument showing the message of circumcision to be completely foolish. It might be interesting to think about how Paul might have phrased this for other legalistic groups. Like for those who teach that you have to get wet to get saved, I wonder if he would have said, "As for those agitators, I wish they would go the whole way and drown themselves!" Or how might you apply this to other examples of legalism?