8 But even if we, or an angel from heaven, should preach to you a gospel contrary to what we have preached to you, he is to be accursed! 9 As we have said before, so I say again now, if any man is preaching to you a gospel contrary to what you received, he is to be accursed (Galatians 1)!
To make his point, Paul writes it twice: the right (only) gospel is so important that preaching or teaching anything contrary to it brings doom and destruction. It also makes no difference who brings that false preaching or teaching—whether they be apostles or even angels. They both receive the same doom and destruction.
Why such severity? Because the gospel was born out of the greatest severity that has ever occurred—the crucifixion of the very Son of God. Without the death of Christ making full (not partial) payment for the sins of man, there is no gospel. Thus, to add anything to it is actually to subtract from it.
Lord, thank You for the severe lengths You took to redeem us, and that we do not need to add anything to it. Help us This Day to find sweet rest in the gospel. Amen.