It is not enough to believe that there is a God, or even to believe that the God of the Bible is the true God. As James tells us: “Thou believest that God is one. Thou doest well. The demons even believe, and tremble” (James 2: 19). No, salvation depends upon believing what God has said.
This is seen very clearly with Abraham. His justification rested on believing what God had testified to him: “And Abraham believed God, and it was reckoned to him as righteousness” (Rom. 4: 3). What had God said? God had told him that even though both he and his wife were old, he would have a son, and his seed would be as numberless as the stars of the heavens (see Gen. 15: 1–6). Looked at through purely natural eyes the fulfilment of this promise seemed impossible, but Abraham believed what God had told him—and as a result God reckoned him as justified. He was justified by faith. Everything pointed to Abraham dying childless, but God had told him otherwise and he believed what God had said. There was nothing that Abraham could do to prove what he had been told and what he had believed, for “faith is [the] substantiating of things hoped for, [the] conviction of things not seen” (Heb. 11: 1, my emphasis).
What was the result of Abraham’s faith? The name of Abraham is renowned through all the world, but like the rest of us, he was a sinner, “for all have sinned, and come short of the glory of God” (Rom. 3: 23). Yet when he believed God, the Scripture tells us that God “reckoned it” (that is, his faith in God’s Word), “to him [as] righteousness” (Gen. 15: 6). Righteousness, instead of sin, was put to his account. Naturally speaking, he was still a sinner, but in the eyes of God he was reckoned as righteous. He was justified. Why? Because he had believed what God had said.Faith, however, cannot stand alone, for “faith without works is dead” (James 2: 26). When Phinehas stood up and executed judgment in Shittim (see Num. 25: 1–15), God says of it that it “was reckoned unto him for righteousness, from generation to generation, for evermore” (Ps. 106: 31). Here the faith of the man is seen in his works, and his faith is proved to be real. A kind of easy belief is rampant today, but “a man is justified on the principle of works, and not on the principle of faith only” (James 2: 24). Thus with Abraham he proved that he believed God by offering “Isaac his son upon the altar” (v21) and so “by works” (v22) his “faith was perfected” (or completed). Faith in God will be “perfected” by bringing forth fruit for God—it cannot be otherwise. A fruitless faith that professes to be the real thing will sooner or later be exposed as seed sown by the wayside, in rocky places, or choked by the thorns (see Luke 8: 5–15).