The Proof of the Assumption

That when God justifies us (that is, when he first declares, and pronounces us to be righteous) he does declare his own righteousness, that he might be just, as Paul speaks (Romans 3:26), and the justifier of him, which believes on Jesus.

And it is the speech of David, that when God declares himself to be just; he declares only the sinfulness of the creature (Psalm 51:4).