18 Therefore hath he mercy on whom he will have mercy, and whom he will he hardeneth. (Romans 9:18 KJV)
William Burkitt’s Commentary
As if Apostle Paul had said, “From these scripture instances we may gather and conclude, that God may without the least injustice magnify his mercy, in sparing and pardoning some sinners, and render his justice glorious in punishing others; yea, in punishing sin with sin, hardening them judicially, who had hardened themselves obstinately.”
Here observe, That God did not harden Pharaoh’s heart by any positive act or influx upon it, by infusing any evil into it; for this would make God the author of sin; but he was hardened by way of judiciary tradition after he had long hardened himself.
First, He was delivered up into the hand of Satan, who deluded him by the magicians counterfeiting the same miracle that Moses did; and this hardened him against the belief of anything that Moses either did or said.
Secondly, He was delivered up to his own lusts, particularly idolatry, ambition, and covetousness; and these hardened Pharaoh’s heart. As an idolater, he was loath to receive a message from the God of Israel, whom he knew not: Who is the Lord, says he, that I should obey him? I know not the Lord.
As an ambitious prince, it went to his very heart, to hear so mean a man as Moses control him in his own dominions, saying, Let the people go, that they may serve the Lord. This enraged him, to hear of any lord over that people but himself; and as a covetous man, he was loath to hear of parting with a people, by whose pains, in making brick, he had so great an income. Thus Pharaoh’s affected hardness was followed by inflicted hardness.
Learn hence, That God does justly deliver that man up to hardness of heart by way of punishment, who has often hardened his own heart against God by repeated acts of sin.
Juste toties cor ejus obduratur in paenam, Quoties ipse cor suum obduravit in culpam. Lightfoot.