A.T. Robertson Commentary Romans 9:18

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

He hardeneth (σκληρυνε). Pharaoh hardened his own heart also (Exodus 8:15,32; 9:34), but God gives men up also (1:24,26,28). This late word is used by the Greek physicians Galen and Hippocrates. See on Ac 19:9. Only here in Paul.