A.T. Robertson Commentary Romans 9:16

"So then it is not of him that willeth, nor of him that runneth, but of God that hath mercy." — Romans 9:16 (ASV)

So then (αρα ουν). In view of this quotation.

It is not of (ου). We must supply εστιν ελεος with ου. "Mercy is not of." The articular participles (του θελοντοσ, του τρεχοντοσ, του ελεωντος) can be understood as in the genitive with ελεος understood (mercy is not a quality of) or as the predicate ablative of source like επιλυσεως in 2 Peter 1:20. Paul is fond of the metaphor of running.