Sin has the amazing ability to slowly harden the heart. It isn’t like liquid nitrogen which will instantly freeze and harden an object. Sin is more like a winter night hovering just at the freezing point that gradually hardens the heart. It starts as a few icy spots on the surface perimeter and slowly creeps across the surface in a thin film. At the same time its numbing affect reaches deeper and its hold becomes stronger and more difficult to break. Eventually the heart is completely hardened.
For this reason the Bible warns us to be on guard against being “hardened by the deceitfulness of sin” (Hebrews 3:13).
Author C.J. Mahaney writes about the hardening effect of sin in his book Humility about the especially dangerous sin of pride:
Sin always has a destructive effect, but often that effect isn’t immediately obvious. Over a period of time, however, where sin is indulged, there’ll be a hardening effect on the soul of a genuinely converted Christian.
To differing degrees we’re all familiar with this hardening effect. Perhaps you gradually find yourself less affected by corporate worship in your local church. Or you’ve recently noticed that your appetite for Holy Scripture has diminished. You may be less sensitive to sin, or your confession of sin is less frequent and lacks sorrow.
The ultimate effect from such hardening by sin is that grace, for the Christian, is no longer amazing. That’s why we need to stay close to the doctrine of sin – because it helps us see the presence of pride and protects us from those hardening effects…and it’s sufficiently potent to put pride to death in our lives by the power of the Holy Spirit…
[Remember] that study alone isn’t sufficient. Along with increased knowledge there must also be grace-motivated application of truth and grace-empowered obedience to truth. Only then will we experience Christ’s liberating power from the sin of pride (pp 93-94).