20 Being most certain that this doctrine (whose integrity We must defend with the help of the Holy Ghost) has been divinely revealed, We repeat these words of the Apostle of the Gentiles: "But even if we or an angel from heaven should preach a gospel to you other than that which we have preached to you, let him be anathema" ( Gal . 1.8).
Source: Ad Sinarum Gentem (Vatican.va)