But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed.
Note 9 at Ga 1:8: Paul's curse in Ga 1:8-9 comprised the strongest statements he made in Scripture against those who opposed his Gospel. Some points of doctrine were negotiable--i.e., nonessential for salvation (see note 1 at 1Co 1:10)--but the grace of God could not be compromised. Paul said that even if an angel appeared preaching any other gospel than what Paul preached, he should be accursed (see note 10 at this verse). This left no room for anyone to preach something different.
Note 10 at Ga 1:8: Here, Paul was stressing the importance of preaching the true message of the Gospel of Christ. If anyone tampers with the true message of the Gospel of grace, Paul stated, "Let him be accursed." The Greek word for "accursed" here is "ANATHEMA," and it means "a person or thing doomed to destruction" (Thayer's Greek-English Lexicon). Today's English Version translates this as "May he be condemned to hell!" This statement will certainly be true of those who have turned away from the good news of Jesus unto another way of salvation.
The counterparts for this word in the Hebrew mean to be appointed to utter destruction and to be completely destroyed. These words were used in Nu 21:3 to describe the children of Israel utterly destroying the Canaanites, and also in Jos 6:16-17 concerning the city of Jericho and its inhabitants being an accursed thing unto the Lord and appointed to utter destruction.