Galatians 5:7-12 (ESV)
You were running well. Who hindered you from obeying the truth?  This persuasion is not from him who calls you.  A little leaven leavens the whole lump.  I have confidence in the Lord that you will take no other view, and the one who is troubling you will bear the penalty, whoever he is.  But if I, brothers, still preach circumcision, why am I still being persecuted? In that case the offense of the cross has been removed.  I wish those who unsettle you would emasculate themselves!
At first glance, it may be difficult to understand why someone would reject a free gift of grace for a demanding law of works. Would you rather win the lottery or work sixty hours a week for forty years? Most of us would probably choose the lottery.
Yet somehow the Galatians, who “were running well” (v.7), are now greatly tempted to abandon Paul’s teaching and embrace circumcision. This error is attractive enough that it could “leaven the whole lump,” or corrupt the entire Galatian Church. It does not come from God (v. 8) and deserves punishment (v. 10), yet the Apostle Paul is persecuted for opposing it. In fact, this issue behind circumcision is at the very center of what makes the cross offensive (v. 11).
So why is the cross so offensive?
The message of the cross is that the Son of God had to die in order to save helpless sinners from themselves. The cross is offensive because it destroys our pride and self-reliance. We’re forced to acknowledge both our wickedness and our complete inability to do anything about it. We’re forced to admit that we are utterly at the mercy of God.
So when the message of circumcision comes to the Galatian Church, this way of earning acceptance before God appeals to their pride. Though painful, circumcision provides the Galatians with a means by which they can boast before God and one another. If one became so bold, he might even demand his rights before God, saying, “I’m circumcised, therefore You HAVE to bless me!”
If salvation is by grace alone, then we cannot boast and we certainly cannot demand anything from God. We can only be thankful and worship.