What is "repentance"?

Repentance is “…radical conversion, a transformation of nature, a definitive turning from evil, a resolute turning to God in total obedience (Mk.1:15; Mt.4:17; 18:3)….  This conversion is once for all.  There can be no going back, only advance in responsible movement along the way now taken.  It affects the whole man, first and basically the center of personal life, then logically his conduct at all times and in all situation, his thoughts, words and acts (Mt. 12:33ff. par; 23:26; Mk.7:15 par.).

“The whole proclamation of Jesus…is a proclamation of unconditional turning to God, of unconditional turning from all that is against God, not merely that which is downright evil, but that which in a given case makes total turning to God impossible….

“It is addressed to all without distinction and presented with unmitigated severity in order to indicate the only way of salvation there is.

“Repentance calls  for total surrender, total commitment to the will of God….  It embraces the whole walk of the new man who is claimed by the divine lordship.  It carries with it the founding of a new personal relation of man to God….  It awakens joyous obedience for a life according to God’s will.”

-- "Theological Dictionary of the New Testament" (Kittel) on 'metanioa' (repentance)