Galatians 3:1-6 (ESV)
O foolish Galatians! Who has bewitched you? It was before your eyes that Jesus Christ was publicly portrayed as crucified.  Let me ask you only this: Did you receive the Spirit by works of the law or by hearing with faith?  Are you so foolish? Having begun by the Spirit, are you now being perfected by the flesh?  Did you suffer so many things in vain—if indeed it was in vain?  Does he who supplies the Spirit to you and works miracles among you do so by works of the law, or by hearing with faith—  just as Abraham “believed God, and it was counted to him as righteousness”?
Note the several ways Paul challenges the heresy that is threatening the Galatian churches:
1) To believe it is to be under some sort of spell. To embrace the Gospel and then add to the glorious work of Christ is to be duped big time. Of course Paul does not think that someone has actually cast a spell upon the Galatians, but his point is that to add anything to the Gospel is to believe some horribly bad stuff.
2) To abandon the Gospel for a system of works is foolish. The Galatians are playing the fool. We may like to blame others for the cleverness of their arguments, the slickness of their presentation, or the attractiveness of the presenters, but in the end we are the ones responsible for what we believe. To fall for a false Gospel after having had it so plainly demonstrated and explained to them, is foolish, no matter how the opposite arguments came to them. People who abandon the Gospel are people who never had it. That is who the real fools are.
3)They are denying the things that they were taught. People who know the Gospel should be able to detect when a denial of it surfaces. Believers have a responsibility to know and study the Gospel and the truths that come out of it. The Gospel we have been taught is for the praise of God’s glory. Anything that puts credit to us for coming to Christ is missing the grand teaching of the Gospel.
We have a glorious Gospel. It should thrill our hearts. We must be on our guard that we never misrepresent it. We must not be so foolish as to as to give it up for stupid counterfeits. And we must grow in our understanding of the Gospel and remember that it will always give God, not us or anyone else, the glory.