κατέναντι οὗ ἐπίστευσεν θεοῦ … καλοῦντος τὰ μὴ ὄντα ὡς ὄντα (4:17)

Says 4:17a quotes Gen 17:5, Πατέρα πολλῶν ἐθνῶν τέθεικά σε, in the words of Swete's LXX (the Hebrew and the name wordplay are the Hebrew Scholar's). Then comes κατέναντι (katenanti, "before, in the presence of") οὗ ἐπίστευσεν θεοῦ. The relative is attracted into the case of θεοῦ, which is drawn into the relative clause: "before the God in whom he believed." Abbott-Smith lists Rom 4:17 for κατέναντι, "before," with θεοῦ.

Says Two participles describe God: τοῦ ζῳοποιοῦντος (zōopoiountos, "who gives life to") τοὺς νεκρούς and καλοῦντος τὰ μὴ ὄντα ὡς ὄντα.

Uncertain καλοῦντος τὰ μὴ ὄντα ὡς ὄντα. This note leans to God's creative call that brings into being what does not exist ("calls into being what does not yet exist," BSB), with ὡς ὄντα giving the result. Creation out of nothing is then the background, not the statement (compare 1 Cor 1:28, τὰ μὴ ὄντα; 2 Macc 7:28, non-canonical). The alternative takes ὡς as "as though": God "speaks of things that are not as though they were," naming the promised nations before they exist (4:17a, 18).

Implies "Who gives life to the dead" prepares for Abraham's and Sarah's deadness (4:19) and for the God who raised Jesus (4:24).