Hard Hearts

Hebrews 3:12-15 See to it, brothers and sisters, that none of you has a sinful, unbelieving heart that turns away from the living God. But encourage one another daily, as long as it is called "Today," so that none of you may be hardened by sin's deceitfulness. We have come to share in Christ, if indeed we hold our original conviction firmly to the very end. As has just been said: "Today, if you hear his voice, do not harden your hearts as you did in the rebellion."

The author of Hebrews is talking to fellow believers ("brothers and sisters") when he warns them to not harden their hearts and turn away in unbelief. I think this is talking to believers who deceive themselves into thinking that they can earn any merit on their own through good works. This creates an attitude of pride and self righteousness that is not pleasing to God. And living that way, under a legalistic religion, is a huge burden to bear. That is why they cannot enter into His "rest". Jesus said that his burden was easy and His yoke was light, because true belief in Jesus means giving up all delusions of self-importance and admitting that we are nothing without him. Humbling ourselves is one of the most difficult things to do, and it must be done daily because we can easily forget, but it is the single most freeing decision in life.