Paul calls faith itself a law
by means of an analogy to the law of works: the law of faith.
This is the only context in which he employs this particular expression. In the original Greek this appears even more clearly, for Paul does not use the definite article here, but rather writes about a law of faith
(as a distinct kind of law, see Romans 3:27). He thereby indicates that faith itself constitutes a new kind of law or rule. This stands in stark contrast to laws that puts us to work to try and merit righteousness, however, as every other law does. Rather, this is a law that prescribes to us the peaceful rest of faith. In other words, by trusting God as he commands us, we refrain from boasting.1
27 Then what becomes of our boasting? It is excluded. By what kind of law? By a law of works? No, but by the law of faith.