In Epistolam D. Pauli Apostoli ad Galatas notae

9 As we have said before, I also say again now, if anyone should preach a Gospel to you other than that which you have received, let him be anathema accursed.

Men are accustomed to think that these things are being amplified too much, or spoken with a certain anger. Therefore, Paul repeats generally: If anyone [preaches] another Gospel. He does the same in chapter 5 regarding drunkenness: About which I have predicted and still predict, etc. The sum of the doctrine is: To have your faith hold such a firm foundation—namely, the immutable truth of God, by which he promises the remission of sins freely in the Gospel—that it is impossible for any creature to shake it. The cause of certainty is the immutable truth of God, which is so certain that every creature who wants to call the Gospel into doubt or change it must be cursed.

For first, God himself, who is above the Angels and all creatures, first brought forth the Gospel in Paradise: The seed of the woman will crush the head of the serpent.

Therefore, the reasoning is the same. Just as Eve, before the fall, if even an Angel from heaven had said, You will not die, should have considered him accursed: so also after the fall, lest we be deceived again (where God promises that we who believe will not die an eternal death, and that because of the death of the Son), the whole human race ought to consider him accursed who dares to call this Gospel into doubt.

He who believes in me will not taste death forever. Likewise: This is the will of the Father, that everyone who sees the Son and believes in him should have eternal life.