Romans 4:17 (As it is written, “I have appointed you a father of many nations”) in the sight of God whom he believed, who gives life to the dead and calls the things not being as being.
Abraham believed God regarding two things: (1) the birth of Isaac, which is related to the God who “calls the things not being as being,” and (2) the offering up and the gaining back of Isaac, which is
related to the God who “gives life to the dead.” Abraham believed such a God and applied Him to his situation. (Rom. 4:17, footnote 1, Recovery Version)
The source of faith is God. He is the One who calls things not being as being and gives life to the dead. (Crystallization-study of the Epistle to the Romans, p.84, by Witness Lee)