But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed: Paul was exhorting the Galatians in the truth of the gospel that they had received from him. It was the gospel and there was no other gospel forth coming. And since there was no other gospel that would come later, the Galatians was not be persuaded by any message or messenger that was contrary to what they had already received.
Paul used a hypothetical illustration about himself, or even an angel from heaven, being accursed if they preached another gospel to the Galatians to demonstrate how persuaded the Galatians should be in the gospel of grace that was originally preached to them.
How persuasive do you think Paul could be? It wass Paul’s job as a preacher of the gospel, as well as the job of all gospel preachers, to be persuasive. (2 Corinthians 5:11) Paul had persuaded some Jews, and a great many devout Greeks and more than a few women. (Acts 17:4) Paul had persuaded people to worship God contrary to the Law of Moses. (Acts 18:13) Paul had persuaded Jews in the synagogue about the kingdom of God. (Acts 19:8) Paul had persuaded a great many people in almost all Asia to follow God. (Acts 19:26) If you are not yet persuaded of his powers of persuasion, then examine the arguments that Paul crafted in Romans. I believe Hebrews is Pauline also. Even according to modern secular standards Paul was a brilliant logician, philosopher and rhetorician. Paul was also equally great by first century Greek standards. Paul was indeed persuasive.
But if that was not enough, Paul stepped it up a notch and included the persuasion powers of angels from heaven. How persuasive do you think that an angel from heaven could be? An angel persuaded a virgin she could conceive a child without the natural contribution of a man. (Luke 1:34-35) An angel convinced a reluctant Joseph who is about to put away his wife for fornication that she had actually conceived of the Holy Spirit and not a man. (Matthew 1:20) An angel convinced Mary Magdalene, and others, at the tomb of Jesus that their dead Messiah had actually been raised from the dead. (Luke 21:1-11) What persuasiveness!
Part of the powerful persuasion of angels is their imposing appearance. They have appearance like lightening and their clothes are white as snow (Matthew 28:2) and they shine with the glory of the Lord (Luke 2:9).
Yet, Paul argued that the Galatian Christians were to be so settled in their conviction of the nature of the gospel that neither he nor an angel could turn them away from it. Obviously an angel from heaven would never preach a gospel other than the true gospel. Paul was using an absurd hyperbole to demonstrate the depth of conviction Christians should have concerning justification by grace.