Jamieson, Fausset and Brown on Romans 9:16

16. So then it is not of him that willeth--hath the inward desire

nor of him that runneth--maketh active effort (compare 1Co 9:24, 26; Php 2:16; 3:14). Both these are indispensable to salvation, yet salvation is owing to neither, but is purely "of God that showeth mercy." See on Php 2:12, 13, "Work out your own salvation with fear and trembling: for it is God which, out of His own good pleasure, worketh in you both to will and to do."