Romans 3:4

‘Let it not be. Yes let God be found true, but every man a liar; as it is written, “That you might be justified in your words, and might prevail when you come into judgment.”'

The thought that God might be unfaithful was inconceivable to Paul. His reply to the expressed doubt is vehement. ‘Let it not be' (or ‘certainly not'). Such a thing could not possibly be true. For the fact was that God would be found true to His faithfulness, even if it meant seeing every man as untrue (a liar).

Indeed the assurance of God's faithfulness was demonstrated in those very oracles which the Jews prided themselves on having received, for they declared that God Himself would be acknowledged as righteous (justified) whatever happened, and would be triumphant when He tried others (or alternately would win the case if He was brought for trial). And that could only mean that what He did was right. The citation is from Psalms 51:4.

The strength of Paul's feeling is brought out by his added statement, ‘let God be found true, but every man a liar'. There was absolutely no truth in the suggestion that God had been found not to be faithful to His promises, even if it meant calling all men liars. Above everything else God was and will be true to what He is.