Ten times Exodus refers to God’s hardening Pharaoh’s heart (e.g., 4:21; 7:3,13), and other times to Pharaoh’s hardening his own heart (e.g., 8:32; 9:34). This does not mean that God actively created unbelief or some other evil in Pharaoh’s heart (see James 1:13), but rather that He withdrew all the divine influences that ordinarily acted as a restraint to sin and allowed Pharaoh’s wicked heart to pursue its sin unabated (see also Romans 1:24,26,28).

Commentary from the MacArthur study Bible, notes for Romans 9:18.