Joshua 11:20 For it was of the Lord to harden their hearts, that they should come against Israel in battle, that he might destroy them utterly, and that they might have no favor, but that he might destroy them, as the Lord commanded Moses.
Hardening the heart against God and His people, and then God hardening the heart, is something of a mystery not readily understood by Christian believers. The devil and the fallen angels hardened their hearts against God and in pride rebelled. The Lord cast them out and they became perpetual enemies, fixed in animosity and evil forever. Adam hardened his heart (as did all of us) and suffered the consequences. Pharaoh hardened his heart himself during the first several plagues; then God hardened his heart.
Unbelieving sinners harden their hearts in unbelief and pride; then God gives them over to the condition and the consequence of their own stubborn choice. So God says in Romans 1:18-32: “Men suppress the truth…did not honor him as God…their foolish hearts were darkened…therefore God gave them up…God gave them up…God gave them up.” If that’s the way you want it, that’s the way you got it. Don’t blame me. You destroyed yourself.
But this holy and just God is also love. He shows it by giving His Son. When we hear the Gospel of the life, death, and resurrection of Christ our hard hearts are softened by overwhelming love, so that by the power of the gospel we receive love, life, and eternal salvation. The Gospel of the Cross changes a hard and resistant heart into a soft and receiving heart. God makes an impression on what He softens. And the end result of Jesus’ victory is that our spiritual enemies are “destroyed utterly.”