let him be under a curse! (literally Let that person be anathema): The Greek word anathema often translates the Hebrew kherem, which in the Old Testament means something dedicated to God for complete destruction (see Leviticus 27:28–29; compare Romans 9:3; 1 Corinthians 12:3; 16:22). Paul called for God's strongest wrath on those who distort the good news about Jesus (Galatians 1:7).
even if we or an angel from heaven: No one is allowed to change the good news (compare 2:6–10, 11–14).