Repentance is when one turns from their sins and wicked ways and puts a submissive faith and trust in Jesus Christ as their Lord and Savior and trusts in Him for their own salvation.
Repentance is not just a guilty feeling, saying you're sorry, or crying tears, although those things might accompany it. True Repentance is not outward reform without inward change.
True repentance is a change of heart towards the sin that one used to love and now hates that results in one turning away from their wicked ways and bearing forth good fruit for God.
Repentance would involve things like this:
A thief who makes any possible restitution and instead of stealing works so that they'll have to give. That would include deleting any pirated content on their computers.
A liar who quits lying and instead uses their tongue for good and for the Truth.
Someone in Authority who once abused their authority but no longer does and instead uses their authority for good.
A cheat who no longer lives by fraud but now acts honestly.
An adulterer/fornicator who quits sleeping around and instead is faithful towards their spouse. Throwing away any pornography they may have as well.
Someone into the occult who wakes up and destroys their witchcraft materials no matter how expensive they may be and turns to serve the God of the Bible.
An idolater who throws away and destroys his former idols.
A person who has realized that they have sinned and fallen short of the glory of God, and has humbly turned from their sins(the definition of repentance), and has submitted their life to the Lordship of Jesus Christ and has trusted in Him(Jesus Christ) as Lord and Savior and obey Him! Notice, I say trust in Jesus Christ, the son of God alone! We don't trust in "mother" Mary, saints, a priest, the pope, the book of mormon, or anything like that.
A Christian is not someone who is merely religious, goes to church, prays prayers, reads bibles, was baptized, or any of that stuff. That stuff is good, but if there's no inward change that results in outward change and good fruit, they are not truly saved.
In addition, despite what many say, God's Grace is NOT a license or a covering for sin, and Christians are not unconditionally eternally secure. A Christian can lose their salvation by renouncing Jesus Christ, especially during times of persecution, by unrepentant willful sin, or by believing a false gospel, or by taking the Mark of the Beast during the 7 year Tribulation period when the Antichrist will be reigning and will require people to worship him and take a mark of loyalty indicating their loyalty to him(the Antichrist) in order to partake in commerce as well to avoid beheading. However, the Bible warns that if ANYONE takes the mark, regardless of the reason, they will spend eternity in the Lake of Fire. Taking 666 is an unforgivable sin. Revelation 14:9-11.
The twelve Apostles, the Apostle Paul, those who have died as martyrs for the faith, the future Tribulation Christians who endure to the end by refusing to take the 666 mark despite the pressures of social rejection, persecution, possible torture, starvation, and the death penalty.