We are warned not to allow ourselves to become hardened, because if we look at the whole concept of hardening in its biblical perspective, we see that something happens to us through repeated sins. Our consciences become seared. The more we commit a particular sin, the less remorse we feel from it. Our hearts are recalcitrant through repeated disobedience.
When God hardens the heart, all He does is step away and stop striving with us. For example, the first time I commit a particular sin, my conscience bothers me. In His grace, God is convicting me of that evil. God is intruding into my life, trying to persuade me to stop this wickedness. If He wants to harden me, all He has to do is to stop rebuking me, stop nudging me, and just give me enough rope to hang myself.
We see in Scripture that when God hardens hearts, He does not force people to sin; rather, He gives them their freedom to exercise the evil of their own desires (James 1:13-15).
Coram Deo: Living in the Presence of God
Pray this prayer with the psalmist David: "Search me, O God, and know my heart; try me, and know my anxieties; and see if there is any wicked way in me, and lead me in the way everlasting" (Ps. 139:23-24).
For Further Study
James 1:13-15: "Let no one say when he is tempted, ‘I am tempted by God'; for God cannot be tempted by evil, nor does He Himself tempt anyone. But each one is tempted when he is drawn away by his own desires and enticed. Then, when desire has conceived, it gives birth to sin; and sin, when it is full-grown, brings forth death."