Repentance
Not groveling, not a feeling, but a turning of the whole life
Repentance is the first word out of the gospel's mouth, and one of the most misread. People hear it as groveling, or as merely feeling bad, or as gritting your teeth and turning over a new leaf by willpower. It is none of those. The word the New Testament uses, metanoia, means a change of mind so deep that it turns the whole direction of a life. It is not the misery before the good news; it is the turning that lets the good news in. And it is not advanced spirituality reserved for the serious. It is the doorway everyone walks through to get in at all.
A turning, not just a feeling
Repentance shows itself in motion, not only in emotion. When Paul summed up his preaching he described it as a turn with visible fruit: men should "repent and turn to God, and do works meet for repentance" (Acts 26:20). John the Baptist demanded the same proof before he would call anyone repentant: "Bring forth therefore fruits meet for repentance" (Matthew 3:8). You can be sorry and change nothing; that is regret. Repentance is sorrow that gets up and walks in a new direction. The feeling may or may not be intense. The turning is the thing.
Godly sorrow and worldly sorrow
Not all sorrow over sin is the same, and the difference decides everything. Paul draws the line cleanly: "godly sorrow worketh repentance to salvation not to be repented of: but the sorrow of the world worketh death" (2 Corinthians 7:10). Two men in the gospel show the two kinds. Peter denied Christ three times and went out and wept bitterly, and his grief ran toward Jesus and he was restored. Judas betrayed Christ and was filled with remorse, and his grief ran away from Jesus, into despair, and it killed him. The same failure, the same sorrow on the surface; but one sorrow drives you back to God and the other drowns you in yourself. Repentance is the first kind: regret pointed home.
The first word of the gospel
It is striking how the New Testament opens its preaching. John the Baptist begins, "Repent ye: for the kingdom of heaven is at hand" (Matthew 3:2). Jesus begins His own ministry with the very same words: "Repent: for the kingdom of heaven is at hand" (Matthew 4:17). He would not soften it, even warning a crowd, "except ye repent, ye shall all likewise perish" (Luke 13:3). And when Peter's hearers at Pentecost asked what they must do, the answer led with it: "Repent, and be baptized every one of you" (Acts 2:38). Repentance is not a fringe demand for the especially bad. It is the front door of the whole house.
And it is itself a gift
Here is the part that keeps repentance from curdling into a work we perform to earn God's favor: repentance is something God gives, not something we manufacture to twist His arm. When the early church saw outsiders turn, they did not congratulate them; they praised God that He had "granted repentance unto life" (Acts 11:18). Paul tells Timothy to deal gently with opponents in hope that "God peradventure will give them repentance to the acknowledging of the truth" (2 Timothy 2:25). And the thing that actually melts a hard heart is not the threat of God but the kindness of God: "the goodness of God leadeth thee to repentance" (Romans 2:4). So repentance and grace are not rivals. Grace is what produces the turning in the first place (see grace).
Repentance and faith: two sides of one turn
You cannot really separate repentance from faith; they are the two halves of a single motion. To repent is to turn from sin and self; to believe is to turn to Christ. Jesus joined them in one sentence: "repent ye, and believe the gospel" (Mark 1:15). Paul preached them as a pair, "repentance toward God, and faith toward our Lord Jesus Christ" (Acts 20:21). A man turning around turns his back on one thing in the very act of turning his face to another. Repentance without faith is mere remorse; faith without repentance is mere agreement. The real turn does both at once.
Repentance is not just the turnstile you pass through once at the start. It is the rhythm of the whole Christian life. The risen Christ called even His churches to repent; John tells believers that "if we confess our sins, he is faithful and just to forgive us our sins" (1 John 1:9). Luther opened the Reformation by saying that when Christ said "repent," He meant the entire life of a believer to be one of repentance, not a single act but a daily turning back toward home. That is not a burden. It is the standing invitation to come back, again, and find the door still open.
Where this lands
Repentance has a bad reputation it does not deserve. It is not God making you crawl before He will have you; it is the turn that finally lets you be had. And heaven does not meet it with a frown. Jesus said the opposite: "joy shall be in heaven over one sinner that repenteth" (Luke 15:7). The party in the prodigal's house begins the moment the son turns for home, while he is still a long way off. So repentance is not the price of the welcome. It is just turning around to discover the Father was already running toward you (see the new birth that follows, and the prayer of a turning heart).
Related: Grace, Born Again, The Sinner's Prayer, Even From Their Own Confessions, and Sanctification. Scripture from the King James Version, linked to BibleHub; the words of Christ are marked in purple.