Matthew Henry on Romans 9:16:
Hence he infers (v. 16), It is not of him that willeth. Whatever good comes from God to man, the glory of it is not to be ascribed to the most generous desire, nor to the most industrious endeavour, of man, but only and purely to the free grace and mercy of God. In Jacob’s case it was not of him that willeth, nor of him that runneth; it was not the earnest will and desire of Rebecca that Jacob might have the blessing; it was not Jacob’s haste to get it (for he was compelled to run for it) that procured him the blessing, but only the mercy and grace of God. Wherein the holy happy people of God differ from other people, it is God and his grace that make them differ. Applying this general rule to the particular case that Paul has before him, the reason why the unworthy, undeserving, ill-deserving Gentiles are called, and grafted into the church, while the greatest part of the Jews are left to perish in unbelief, is not because those Gentiles were better deserving or better disposed for such a favour, but because of God’s free grace that made that difference. The Gentiles did neither will it, nor run for it, for they sat in darkness, Mt. 4:16. In darkness, therefore not willing what they knew not; sitting in darkness, a contented posture, therefore not running to meet it, but anticipated with these invaluable blessings of goodness. Such is the method of God’s grace towards all that partake of it, for he is found of those that sought him not (Isa. 65:1); in this preventing, effectual, distinguishing grace, he acts as a benefactor, whose grace is his own. Our eye therefore must not be evil because his is good; but, of all the grace that we or others have, he must have the glory: Not unto us, Ps. 115:1.
Source: http://www.puritanboard.com/showthread.php/90719-Rejection-of-the-doctrines-of-Grace, Comment #30