How can I not have a hardened heart?

from the old testament

• Consistent rebellion against God hardens the heart. God had Zechariah prophesey against His people who had "turned a stubborn shoulder" to God's commands and "made their hearts diamond-hard" (Zechariah 7:11-12; cf. Deuteronomy 10:16).
• Hardened hearts may also be a form of Divine judgment for persistent sin. Through the psalmist, God says, “But my people did not listen to my voice; Israel would not submit to me. So I gave them over to their stubborn hearts,to follow their own counsels" (Psalm 81:11-12).
• Meditating on God's word can guard against a hardened heart. The psalmist describes the "insolent" as having hearts that are "unfeeling" as contrasted to those who "delight" in God's law (Psalm 119:69-70).
  
• We can ask God to protect us against a hardened heart, as David did: "Search me, O God, and know my heart! Try me and know my thoughts! And see if there be any grievous way in me, and lead me in the way everlasting!" (Psalm 139:23-24).

from the new testament

• A hardened "heart" is a way of referring to lack of spiritual perception. More than once, Jesus identified that condition in His disciples (Luke 24:25, 32). One example is when they wondered at how to obtain food after having already witnessed Jesus' miraculous feeding of the four thousand (Mark 8:1-21). Their concern for where to get food showed a lack of faith, a hardness of heart.
• Paul suggests that those who live in sin can develop seared consciences, another way of saying hardened hearts (1 Timothy 4:1-2).
• Divine judgment against those who "suppress the truth" (Romans 1:18) can lead to a hardened heart. Paul speaks of such people who "became futile in their thinking, and their foolish hearts were darkened" (Romans 1:21). The way out of this is to hold fast to the truth.
• Meditating on God's word helps us discern "the thoughts and intentions of the heart" (Hebrews 4:12). Our wayward hearts can lead us astray, but God's word grounds us.

implications for today

Lie often enough and you may start to believe yourself. That's the nature of the human heart. The more we sin, the more comfortable we get with it. The distance between using the company card for a personal lunch and embellezing money from the company becomes a matter of opportunity rather than conscience. Flirtatious texts with a coworker that you hide from your spouse become a full-blown affair. Our hearts become calloused, numb to sin.

How do we guard against that? We need to cultivate habits that remind us of our dependence on God. Often, we live as if we are good enough. But when we recognize our sins and live lives attentive to His Spirit, we keep our hearts tender and listening to Him. God's Word is an excellent mirror into our lives. It reveals our sin and exposes our complacencies and self-righteousness. When we are convicted, we also guard against hard hearts by confessing sin quickly instead of excusing it. Prayer is also the lifeline that keeps us near God while living in godly community is another tangible way that keeps our hearts tender toward God and His conviction.

All this is important because a soft heart is usually not lost all at once but slowly neglected through compromise, distraction, pride, and secret sin. The longer we justify sin, the quieter our conscience becomes and the easier it is to drift from God without noticing. But when we regularly bring our hearts before the Lord, He sharpens our discernment, renews our sensitivity to sin, and keeps us walking in truth. What we cannot do is casually play with sin and assume we will stay spiritually healthy because the sins we tolerate today often become the chains that enslave us tomorrow.

A hard heart rarely appears overnight—it is formed one compromise at a time. That is why we must guard our hearts before sin becomes comfortable, because the heart that stops listening to God today may eventually stop hearing Him altogether.