Albert Barnes Commentary Romans 9:18

"So then he hath mercy on whom he will, and whom he will be hardeneth." — Romans 9:18 (ASV)

Therefore he has mercy, and so on. This is a conclusion stated by the apostle as the result of all the argument.

Whom he will he hardens. 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 'hardens' 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. See Barnes' notes on John 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.