To repent is to change your mind. Considered morally, it is to change your mind regarding your chosen ultimate end in life, which necessarily determines your way of life. That is, to change it by forsaking the choice of your own supreme happiness, to then choose impartial universal happiness, which is the will of God. A change from selfishness to love.
One may also say biblical repentance, regarding salvation, is turning from your evil way - by turning away from and forsaking your old unclean and corrupt purpose of heart, and consecrating your whole heart, mind and strength to the known will of God.
The following is a selection of examples of calls to repentance from the Old Testament writings:
“Turn now from your evil ways and your evil deeds.” [From Zechariah 1]
“Turn now everyone from his evil way, amend your doings, and do not go after other gods to serve them.” [From Jeremiah 35]
“Return to the LORD with all your hearts, then put away the foreign gods.. from among you, and prepare your hearts for the LORD, and serve him only.” [From 1 Samuel 7]
“Wash yourselves, make yourselves clean; put away the evil of your doings from before my eyes. Cease to do evil, learn to do good.” [From Isaiah 1]
“Let the wicked forsake his way, and the unrighteous man his thoughts.” [Isaiah 55]
In the 18th chapter of Ezekiel, God says, “Repent, and turn from all your transgressions, so that iniquity will not be your ruin. Cast away from you all the transgressions which you have committed, and get yourselves a new heart and a new spirit.” And in the 33rd chapter of Ezekiel, God says, “As for the wickedness of the wicked, he shall not fall because of it in the day that he turns from his wickedness.”
From the 2nd chapter of Joel - “'Now, therefore,' says the LORD, 'Turn to me with all your heart, with fasting, with weeping, and with mourning.' So rend your heart, and not your garments; return to the LORD your God, for he is gracious and merciful, slow to anger, and of great kindness; and he relents from doing harm.”
A great example of repentance in the Old Testament writings is found in the book of 'Jonah' - “'Arise, go to Nineveh, that great city, and cry out against it; for their wickedness has come up before me.' ...Jonah arose and went to Nineveh, according to the word of the LORD.. Then he cried out and said, 'Yet forty days, and Nineveh shall be overthrown!' ..Then God saw their works, that they turned from their evil way; and God relented from the disaster that he had said he would bring upon them, and he did not do it.” Their 'works' were: covering themselves in sackcloth, crying mightily to God and every one turning from his evil way. Later Jesus would say, “The men of Nineveh will rise up in the judgment with this generation and condemn it, because they repented at the preaching of Jonah; and indeed a greater than Jonah is here.”
In the Gospels we read of John the Baptist preaching in the wilderness of Judea, and saying, “Repent, for the kingdom of heaven is at hand!” And upon seeing many of the Pharisees and Sadducees coming to his baptism, he said to them, “Brood of vipers! Who warned you to flee from the wrath to come? Therefore bear fruits worthy of repentance.. every tree which does not bear good fruit is cut down and thrown into the fire.”
From the time Jesus began to preach, he said, “Repent, for the kingdom of heaven is at hand.” He also rebuked the cities in which most of his mighty works had been done, “because they did not repent.” After Jesus called the twelve to himself, and began to send them out two by two, we read that “they went out and preached that people should repent.” Jesus is also recorded as saying, “I did not come to call the righteous, but sinners, to repentance.” And, “There will be more joy in heaven over one sinner who repents than over ninety-nine just persons who need no repentance.. there is joy in the presence of the angels of God over one sinner who repents.”
Before his ascension, Jesus said, “Thus it is written, and thus it was necessary for the Christ to suffer and to rise from the dead the third day, and that repentance and remission of sins should be preached in his name to all nations, beginning at Jerusalem.”
During Pentecost crowds in Jerusalem “were cut to the heart”, and said to Peter and the rest of the apostles, “Men and brethren, what shall we do?” Then Peter said to them, “Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins.” Peter also said “Those things which God foretold by the mouth of all his prophets, that the Christ would suffer, he has thus fulfilled. Repent therefore and be converted, that your sins may be blotted out.” In his second letter, Peter wrote, “The Lord is longsuffering toward us, not willing that any should perish but that all should come to repentance.”
In the 26th chapter of Luke's 'Acts of the Apostles', we read that Paul told King Agrippa, “I was not disobedient to the heavenly vision, but declared first to those in Damascus and in Jerusalem, and throughout all the region of Judea, and then to the Gentiles, that they should repent, turn to God, and do works befitting repentance.”
Paul says to the Romans, “Do you despise the riches of his goodness, forbearance, and longsuffering, not knowing that the goodness of God leads you to repentance?” And to the Corinthians, “Godly sorrow produces repentance leading to salvation, not to be regretted; but the sorrow of the world produces death.”
Finally, in the book of 'Revelation', Jesus tells the seven churches of Asia,
“Repent and do the first works, or else I will come to you quickly and remove your lampstand from its place—unless you repent.”
And,
“I have not found your works perfect before God. Remember therefore how you have received and heard; hold fast and repent.”