Romans 3:26 To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus.
27 Where is boasting then? It is excluded. By what law? of works? Nay: but by the law of faith.
This could be called the First Spiritual Law: the Law of Faith.
This is contrasted to Moses' Laws, which require that we do something for the law to take effect. But salvation in the Mosaic Law also requires faith.
The Law of Faith is radically different. It requires that we believe something for the law to take effect. Thus "Believe on the Lord Jesus Christ and thou shalt be saved." Believe in the care of God, believe in the promises of God, believe in the Words of God. Faith may result in action, but the faith itself is the operative power. Thus when Mary said "Be it done unto me according to Thy Word," she didn't have to do anything for Jesus to be conceived. Everything was done by God. Similarly the thief on his cross at Calvary, just believed that Jesus was the King coming into His Kingdom, and he was saved. There was nothing he had to do.
Faith plus nothing equalled everything in both cases.