Calvin had this to say about Galatians 1:
He calls it
another gospel, that is, a gospel different from the true one. And yet the false apostles professed that they preached the gospel of Christ; but, mingling with it their own inventions, by which its principal efficacy was destroyed, they held a false, corrupt, and spurious gospel.
He charges them with the additional crime of doing an injury to Christ, by endeavoring to subvert his gospel. Subversion is an enormous crime. It is worse than corruption. And with good reason does he fasten on them this charge. When the glow of justification is ascribed to another, and a snare is laid for the consciences of men, the Savior no longer occupies his place, and the doctrine of the gospel is utterly ruined.
To know what are the leading points of the gospel, is a matter of unceasing importance. When these are attacked, the gospel is destroyed.
...he declares that the doctrine which he had preached is the only gospel, and that the attempt to set it aside is highly criminal. But then he was aware, the false apostles might object: "We will not yield to you in our desire to maintain the gospel, or in those feelings of respect for it which we are accustomed to cherish." Just as, at the present day, the Papists describe in the strongest terms the sacredness with which they regard the gospel, and kiss the very name with the deepest reverence, and yet, when brought to the trial, are found to persecute fiercely the pure and simple doctrine of the gospel.
Accordingly, Paul does not rest satisfied with this general declaration, but proceeds to define what the gospel is, and what it contains, and declares boldly that his doctrine is the true gospel; so as to resist all further inquiry. Of what avail was it to profess respect for the gospel, and not to know what it meant?