4. God forbid—literally, "Let it not be," that is, "Away with such a thought"—a favorite expression of our apostle, when he would not only repudiate a supposed consequence of his doctrine, but express his abhorrence of it. "The Scriptures do not authorize such a use of God's name as must have been common among the English translators of the Bible" [Hodge].
yea, let God be—held
true, and every man a liar—that is, even though it should follow from this that every man is a liar.
when thou art judged—so in Ps 51:4, according to the Septuagint; but in the Hebrew and in our version, "when thou judgest." The general sentiment, however, is the same in both—that we are to vindicate the righteousness of God, at whatever expense to ourselves.