6. I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel:

[Paul] commences by administering a rebuke, though a somewhat milder one than they deserved; but his greatest severity of language is directed, as we shall see, against the false apostles. He charges them with turning aside, not only from his gospel, but from Christ; for it was impossible for them to retain their attachment to Christ, without acknowledging that he has graciously delivered us from the bondage of the law. But such a belief cannot be reconciled with those notions respecting the obligation of ceremonial observance which the false apostles inculcated. They were removed from Christ; not that they entirely rejected Christianity, but that the corruption of their doctrines was such as to leave them nothing more than an imaginary Christ.

Thus, in our own times, the Papists, choosing to have a divided and mangled Christ, have none, and are therefore “removed from Christ.” They are full of superstitions, which are directly at variance with the nature of Christ. Let it be carefully observed, that we are removed from Christ, when we fall into those views which are inconsistent with his mediatorial office; for light can have no fellowship with darkness.

On the same principle, 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. By using the present tense, (“ye are removed”) he appears to say that they were only in the act of failing. As if he had said, “I do not yet say that ye have been removed; for then it would be more difficult to return to the right path. But now, at the critical moment, do not advance a single step, but instantly retreat.”

From Christ, who called you by grace. Others read it, “from him who called you by the grace of Christ,” understanding it to refer to the Father; but the reading which we have followed is more simple. When he says that they were called by Christ through grace, this tends to heighten the criminality of their ingratitude. To revolt from the Son of God under any circumstances, is unworthy and disgraceful; but to revolt from him, after being invited to partake salvation by grace, is more eminently base. His goodness to us renders our ingratitude to him more dreadfully heinous.

So soon. When it is considered how soon they had discovered a want of steadfastness, their guilt is still further heightened. A proper season, indeed, for departing from Christ cannot be imagined. But the fact, that no sooner had Paul left them than the Galatians were led away from the truth, inferred still deeper blame. As the consideration of the grace by which they had been called was adduced to aggravate their ingratitude, so the circumstance of the time when they were removed is now adduced to aggravate their levity. (Online source)

John Calvin