Romans 3:4

Impossible! Rather let God be seen to be true though all mankind should be proved false, even as the Psalmist looked upon his own sin as serving to enhance the triumph of God’s justice. Speaking of that justice for the moment as if it could be arraigned before the bar of a still higher tribunal, he asserts its absolute and complete acquittal.

That thou mightest be justified. — Strictly, in order that, here as in the Hebrew of the Psalm. Good is, in some way inscrutable to us, educed out of evil, and this is clearly foreseen by God, and forms part of His design, though so as not to interfere with the free-will of man. Religion assumes that the two things, free-will and omnipotence, are reconcilable, though how they are to be reconciled seems an insoluble problem. The same difficulty attaches to every system but one of blank fatalism and atheism. But the theory of fatalism if logically carried out would simply destroy human society.

Psalms 51, in which the quotation occurs, is commonly (in accordance with the heading), though perhaps wrongly, ascribed to David after his sin with Bathsheba. The effect of this sin is to throw out into the strongest relief the justice of the sentence by which it is followed and punished. The original is, “That thou mightest be just in thy speaking; that thou mightest be pure in thy judging.” St. Paul adopts the rendering of the LXX., who make the last word passive instead of active, thus making it apply, not to the sentence given by God, but to the imaginary trial to which by a figure of speech that sentence itself is supposed to be submitted.