God tells us there is something we must do before He will apply the sacrifice of Christ to pay the penalty of our sins. We must demonstrate that we no longer wish to continue in the way of life which leads to eternal death. We do so by changing our way of living. That change of direction is called "repentance" in biblical terminology.
But what, exactly, do we repent of? Sin!
Actions and deeds that are contrary to God's law are simply defined as "sin." Sin is the violation or transgression of any of God's great spiritual laws. Notice the definition of sin in the Bible: "Whosoever commits sin transgresses also the law: for sin is the transgression [breaking] of the law" (I John 3:4).
Sin, however, is not always a wrong deed. Sometimes we sin by not doing what is right or good. "Therefore to him that knows to do good, and does it not, to him it is sin" (James 4:17).
To repent of sin, then, simply means to "change direction." We turn from the way of self-indulgence and "get" to the way of "give." We stop serving the inordinate lusts of our own flesh and begin to serve others. We turn from selfishness to selflessness.
When we begin demonstrating our willingness to change and ask God to forgive our past way of life, He applies the sacrifice of Christ on our behalf. We are then freed from the crushing guilt of sin and its penalty. We are forgiven and our conscience is cleared. The penalty of sin - eternal death - no longer hangs over our head.
Paul explained it this way: "How much more shall the blood of Christ, who through the eternal Spirit offered himself without spot to God, purge your conscience from dead works to serve the living God?" (Hebrews 9:14).