God forbid: yea, let God be true, but every man a liar; as it is written, That thou mightest be justified in thy sayings, and mightest overcome when thou art judged.
Note 6 at Ro 3:4: This is the first of ten times that Paul used the expression "God forbid" in the book of Romans (this verse, 3:6, 31; 6:2, 15; 7:7, 13; 9:14; 11:1, and 11). He also used this expression four other times in his other epistles (1Co 6:15; Ga 2:17, 3:21, and 6:14).
The Greek words that are used here for "God forbid" are "ME GINOMAI" meaning "let it not be...God forbid...far be it" (Vine's Expository Dictionary) and express emphatic denial of the false conclusion that someone might draw from his teaching.
Note 7 at Ro 3:4: This phrase "let God be true, but every man a liar" is given in response to the question of Ro 3:3 (see note 5 at that verse). Paul was stating that God and His promises are always true even if people don't believe Him. However, other applications of this truth can benefit us greatly.
When anyone or anything contradicts a promise made to us in God's Word, we need to reckon God to be true and that person or thing to be lying. We need to believe that what God's Word says about our prosperity is true (2Co 8:9 and 3Jo 2), instead of what our checkbooks say. We need to believe that we were healed by His stripes (Isa 53:5, Mt 8:17, and 1Pe 2:24), instead of believing what the x-rays show. In every aspect of our lives, we need to believe God's Word above what we see or hear.
Note 8 at Ro 3:4: This is a quotation from David out of Ps 51:4 when he was repenting of his sin with Bathsheba and the murder of her husband. David was admitting his sinfulness and proclaiming God's complete justification in judging his sin in whatever way He saw fit. David's sin had not made God unholy; it made David unholy. In his sinfulness, David saw the holiness of God more clearly than ever.
This is what Paul was drawing from this Old Testament passage. He was saying that in a similar manner, God retained His holiness even when His people were unholy. It was the Jews, not God, who suffered from not believing God's Word.