“That is why it depends on faith, in order that the promise may rest on grace and be guaranteed to all his offspring—not only to the adherent of the law but also to the one who shares the faith of Abraham, who is the father of us all, as it is written, “I have made you the father of many nations”—in the presence of the God in whom he believed, who gives life to the dead and calls into existence the things that do not exist.” -Romans 4:16–17
In verse 17, Paul supplies Scriptural support for the argument he is wrapping up (verse 16) and recounts the covenant God made with Abraham.
“No longer shall your name be called Abram, but your name shall be Abraham, for I have made you the father of a multitude of nations.” -Genesis 17:5
Abraham is the father of us all, Paul argues, because God established the covenant with Abraham by faith which made him the father of not only the Jews (those who would receive the Law some 400 years later) but also of the Gentiles who’s share in the faith of Abraham.
In the final clause, Paul highlights the content (and the character of the content) of Abraham’s faith, God. Abraham neither trusted in his own merits, nor in the gods of the Chaldeans, but in the God who gives life to the dead and calls into existence things that do not exist. This way of describing God, obviously refers to the content of the promise God made to Abraham in Genesis 17, namely that Sarah who was 90 (far beyond childbearing age) would conceive and give them a son who would make Abraham the father of many nations.