I will have mercy - This is said by God when he declared expressly that he would make all his goodness pass before Moses Exodus 33:19, and when, therefore, it was regarded, not as a proof of stern and inexorable justice, but as “the very proof of his benevolence,” and the highest which he thought proper to exhibit.  When people, therefore, under the influence of an unrenewed and hostile heart, charge this as an unjust and arbitrary proceeding, they are resisting and perverting what God regards as the very demonstration of his benevolence.  The sense of the passage clearly is, that he would choose the objects of his favor, and bestow his mercies as he chose.  None of the human race deserved his favor; and he had a right to pardon whom he pleased, and to save people on his own terms, and according to his sovereign will and pleasure. (Dr. Albert Barnes)

This is produced, in favor of special, particular, and personal election, and to clear it from any charge of unrighteousness; and by it, it appears, that God bestows his grace and mercy in time, on such persons he has willed and determined from all eternity to bestow it; this, is clear from hence, for since all this is dependent on his will, it must be as this was his will from eternity, seeing no new will can possibly arise in God, God wills nothing in time, but what he willed before time; that this grace and mercy are shown only to some persons, and that the only reason of this is his sovereign will and pleasure, and not the works and merits of men; wherefore since this grace and mercy rise out of his own free good will and pleasure, and are by no means the creature's due, it most clearly follows, that God in determining to bestow his grace and mercy, and in the actual doing of it, whilst he determines to deny it, and does deny it to others, cannot possibly be chargeable with any unrighteousness. (Dr. John Gill)

The apostle, having asserted the true meaning of the promise, comes here to maintain and prove the absolute sovereignty of God, in disposing of the children of men, with reference to their eternal state.  And herein God is to be considered, not as a rector and governor, distributing rewards and punishments according to his revealed laws and covenants, but as an owner and benefactor, giving to the children of men such grace and favor as he has determined in and by his secret and eternal will and counsel: both the favor of visible church-membership and privileges, which is given to some people and denied to others, and the favor of effectual grace, which is given to some particular persons and denied to others. (Mathew Henry)