Romans 9:16 : So then it is not of him that willeth, nor of him that runneth, but of God that hath, mercy. This is a conclusion, not so much from what is said in Romans 9:15 as from the whole scope of Paul's argument. In bringing to maturity his plans to bless all nations through Abraham's seed, God had followed the counsel of his own will. The promised seed was Christ, and not the Jewish nation, as the Jews thought. These blessings would be bestowed according to God's good pleasure, and not according to any racial dinstinctions. The Jews willed that it should be otherwise.
They would have no Gentile blessed unless he became circumcised and kept the law of Moses. Their striving earnestly, for so the word run implies, could not defeat the purpose of God, any more than Isaac and Esau could defeat God's purpose to bless Jacob. The working out of God's plan through the men Paul mentioned had nothing to do with their personal salvation. Someone had to be selected through whose seed the world would be blessed. God selected Abraham. Of Abraham's-- sons, one had to be an heir of the promise as the progenitor of the Messiah, the promised seed God selected Isaac; and so also with reference to Isaac's sons, Jacob and Esau Jacob was selected.
However, since the way of salvation through Christ has been opened, a man's own will is the deciding factor in his salvation. "And he that is athirst, let him come: he that will, let him take the water of life freely."