A.T. Robertson Commentary Galatians 1:8

"But though we, or an angel from heaven, should preach unto you any gospel other than that which we preached unto you, let him be anathema." — Galatians 1:8 (ASV)

If we (εαν ημεις). Condition of third class (εαν and aorist middle subjunctive ευαγγελισητα). Suppose I (literary plural) should turn renegade and preach "other than" (παρ' ο), "contrary to that which we preached." Preachers have turned away from Christ, alas, and preached "humanism" or some other new-fangled notion. The Jews termed Paul a renegade for leaving Judaism for Christianity. But it was before Paul had seen Christ that he clung to the law. Paul is dogmatic and positive here, for he knows that he is standing upon solid ground, the fact of Christ dying for us and rising again. He had seen the Risen Jesus Christ. No angel can change Paul now.

Let him be anathema (αναθεμα εστω). See on 1 Corinthians 12:3 for this word.