The gist of the matter is that Paul attacks those who, even having accepted Christ with faith, still are prone to practice the works of Law. Now, having accepted Christ means that we have relegated the entire element of our sinfulness to the fire of His grace, and so nothing is left in us that is not in the process of redemption and, since nothing is left, then nothing is also needful save the fire of Christ's grace that acts with our co-action within us, recreating us into a "new creation" (Gal. 6:15).
But if one still rebuilds the works of Law, he will calumniate Christ falsely accusing Him in that He and His grace aren't enough to take away the entire element of sinfulness and falledness in us, but still something of this element remains unhealed - even with the fire of Christ's grace working in us - and for regarding and entertaining or checking this allegedly remaining element, we need to do again works of the Law that we have abandoned after reception of Christ with a hope that He would heal it; but since He failed, so we are restoring the deeds of the Law, and Christ helps us to do those deeds having become alongside with us a deacon or servant of the sin that He couldn't heal by His grace.
It is because of the utter preposterousness and madness of such a sacrilegious supposition that Paul says or even shouts, being thoroughly appalled: μὴ γένοιτο - "Let it not be!".