16 So then it is not of him that willeth, nor of him that runneth, but of God that sheweth mercy. (Romans 9:16 KJV)
William Burkitt’s Commentary
As if Apostle Paul had said, “The foregoing instances abundantly show, That it is not of him that willeth; for Abraham willed that Ishmael might live to be partaker of the blessing promised to his seed, when he said, O that Ishmael might live before thee! Ge 17:18 Nor is it of him that runneth: for when Esau ran to fetch venison for his father so that he might receive the blessing, Ge 25:28, the wisdom of God saw fit to have it otherwise and to confer the blessing upon Jacob; but it is of God that showeth mercy that anyone is chosen to be the seed to which the promise made to Abraham belongs and so to be his church and people.”
Learn hence, That it is of God’s mere grace and mercy that any sinners are called and admitted to the privilege of justification and adoption upon any terms and conditions whatsoever. The reason why the sinful and unworthy Gentiles were called to be a people who were not a people, while the Jews were left out, and cast off for their obstinate unbelief, was not because the Gentiles were either more worthy or more willing, but from God’s discriminating grace and mercy: It is not of him that willeth, but of God that sheweth mercy.