Albert Barnes on Romans 9:18

Therefore hath he mercy, etc. This is a conclusion stated by the apostle as the result of all the argument.

Whom he will he hardeneth. This is not stated in what the Scripture said to Pharaoh, but is a conclusion to which the apostle had arrived, in view of the case of Pharaoh. The word hardeneth means only to harden in the manner specified in the case of Pharaoh. It does not mean to exert a positive influence, but to leave a sinner to his own course, and to place him in circumstances where the character will be more and more developed. Jn 12:40. It implies, however, an act of sovereignty on the part of God in thus leaving him to his chosen Course, and in not putting forth that influence by which he could be saved from death. Why this is, the apostle does not state. We should, however, not dispute a fact everywhere prevalent; and should have sufficient confidence in God to believe that it is in accordance with infinite wisdom and rectitude.