Note 9 at Gal. 6:17: Paul had already presented all his arguments to the Galatians. He's had it with them. They can either accept the gospel or reject it, but he won't compromise. They don't need to trouble him with any more quarrels over this issue. He's branded forever with the marks of the Lord Jesus.

This is probably a reference to the custom of the day where slaves who chose to stay with their master were permanently branded in their flesh. Once this happened, they could never go free (Ex. 21:6; Dt. 15:17). The scars in Paul's flesh that he suffered as a result of preaching the gospel were as this brand to him. He would never deviate from the truth of the gospel.