Faith: by faith Abraham is “the father of many nations”; by faith we are the descendants of Abraham. And vv. 16-25 focus on Abraham’s faith.
Wright in the New Interpreter’s Bible Commentary observes (1) Paul’s description of Abraham’s faith (vv.17-21) contrasts with the human failure described in 1:20-27, e.g., Abraham recognizing God’s power, giving glory to God, etc.; so “In vv.18-22 he demonstrates that when Abraham believed this promise he was exemplifying what it meant to be truly human, in contrast to the human disintegration in 1:18-3:20”; (2) “’Faith’, for Paul, is never a thing in itself, but is always defined, as Rom 4:16-22 makes clear, in relation to the God in whom trust is placed. The purpose of a window is not to cover one wall of the house with glass, but to let light in and to let the inhabitants see out.”
I do wonder about Paul’s reading of Genesis 15:1-6. “No distrust made him waver”—but in the next chapter (Gen 16) he agrees to Sarah’s suggestion to have a child by Hagar, and in Gen 20 he’s (again) passing Sarah off as his sister. Paul’s reading makes more sense if he’s also thinking of the binding of Isaac (Gen 22), so perhaps he’s working from an interpretive tradition that’s already associated the two stories.
“…the God in whom he believed, who gives life to the dead and calls into existence the things that do not exist” (v.17). That belongs closer to the center of my image of God. Ezekiel’s valley filled with very dry bones (Ezek 37) too often mirrors our world; for Abraham and Sarah’s God that’s not where stories end, but where they start.