Ps. 51:3-6 For I recognize my shameful deeds--they haunt me day and night. Against you, and you alone, have I sinned. I have done what is evil in your sight. You will be proved right in what you say, and your judgment against me is just. For I was born a sinner--yes, from the moment my mother conceived me. But you desire honesty from the heart, so you can teach me to be wise in my innermost being.
Repentance is essential because God is holy and only he who is holy can draw near to God and have fellowship with Him.
Repentance is facing up to what we have done. It is not sugarcoating the truth. It is knowing we have sinned and doing something about it. When we sin, we sin against God. It is knowing that we have to answer to God for what we have done. It is knowing that the guilt and shame we feel is warranted. It is admitting that God is justified in condemning us for our sin. It is knowing that God alone is the one we have to answer to for our sin.
Sometimes repentance also means we have to go to someone we have sinned against and humbly admit our sin and seek forgiveness. We cannot know God's peace while maintaining broken relationships with man. We must swallow our pride and seek to be reconciled with others before we can really be reconciled with God. When reconciliation is not possible due to the other person's stubbornness of heart, we can have peace knowing we have tried. To not try is disobedience against God.
Repentance is absolutely essential if we want a heart that is right with God.