John 12:40

He has blinded their eyes and made their hearts insensible, to prevent them seeing with their eyes and understanding with their hearts and turning for me to cure them.

Romans 9:18-20

Thus God has mercy on anyone just as he pleases, and he makes anyone stubborn just as he pleases.

2 Thessalonians 2:11-12

Therefore God visits them with an active delusion, till they put faith in falsehood,

Treasury of Scripture Knowledge

Summary

O LORD, why hast thou made us to err from thy ways, and hardened our heart from thy fear? Return for thy servants' sake, the tribes of thine inheritance.

And hardened

Bible References

Why

2 Thessalonians 2:11
Therefore God visits them with an active delusion, till they put faith in falsehood,

And hardened

John 12:40
He has blinded their eyes and made their hearts insensible, to prevent them seeing with their eyes and understanding with their hearts and turning for me to cure them.
Romans 9:18
Thus God has mercy on anyone just as he pleases, and he makes anyone stubborn just as he pleases.