Resistance is the will’s refusal of the light and grace God offers; hardening is the settled condition such refusal produces.
God’s drawing can be resisted, and Scripture is full of those who would not come — who loved darkness rather than light and stiffened themselves against the Spirit. Persistent resistance hardens: the more the light is refused, the less it is felt, until the heart is set.
This hardening is judicial as well as personal — God gives people over to the refusal they have chosen — yet it never falls on the willing. The call is always, “To day, if ye will hear his voice, harden not your hearts.”
See in Scripture: Acts 7:51; John 3:19–20; Hebrews 3:7–8, 15; Romans 1:24