What is repentance?

Repentance is a change of mind, or will, based upon godly sorrow and resulting in a manner of life directed by Christ.

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2 Corinthians 7:10-11 — For the sorrow that is according to {the will of} God produces a repentance without regret, {leading} to salvation, but the sorrow of the world produces death.
For behold what earnestness this very thing, this godly sorrow, has produced in you: what vindication of yourselves, what indignation, what fear, what longing, what zeal, what avenging of wrong! In everything you demonstrated yourselves to be innocent in the matter.

Acts 26:20 — but {kept} declaring both to those of Damascus first, and {also} at Jerusalem and {then} throughout all the region of Judea, and {even} to the Gentiles, that they should repent and turn to God, performing deeds appropriate to repentance.