8 But even if we or an angel from heaven should preach a gospel contrary to the one we preached to you, let him be under a curse! -Galatians 1:8 BSB
The word “we” includes Paul, his companions, and the 12 Apostles.
The phrase “an angel from heaven” is used here as a worst-case scenario, even though it’s an impossibility for a good angel to deceive you. Paul is making the point about the seriousness of believing in a false gospel. If a person believes in a false gospel with a false Jesus, the person will pay for it, for all eternity, in the flames of hell.
Also, we can examine very well, if in a vision or dream, an angel comes to you, you can test his words through God’s Word to see if indeed an angel from the Lord was sent to you or a demon masquerading as an angel of light came to deceive you.
The phrase “should preach a gospel contrary to the one we preached to you” refers to a false gospel that originates from demons called “teachings of demons (1 Timothy 4:1).”
Anyone who preaches a false gospel, a demonic doctrine, will be under God’s curse. The Greek word for “under a curse” is anathema, a very strong word to suggest a person who has been devoted to destruction through God’s judgment.