What is "repentance"?
It is the last word - the sum total of God's call to men as sinners - proclaimed in the Torah and by the last of the Hebrew prophets, John the Baptist.
In continuity with the prophets of Israel in the Hebrew Scriptures, and in the tradition of John the Baptist, it is the first word of God's call to men as sinners proclaimed by the Son of God, Jesus Christ, carried forward by the apostolic and early Church witnesses; it is the message that continue today, in these the last days since Pentecost, the divine imperative, never watered down through the ages up to today.
It is the call of God to men to abandon their own way of life and live to God's will. Repentance is the walk of Man's return to the Garden of Eden.
It is the divine call that has an eye to the Kingdom of God, that is, God's all-encompassing glory exposed and ruling over all the earth with absolute moral goodness. Consequently, it implies both warning and promise: the promise of a new life in a new age for a new people in a new earth centered in a new city. And the warning of divine retribution against men as sinners who oppose his rule, refusing to align themselves now - not tomorrow, but now - with the imminent and inevitable reign of God's Son, Jesus Christ over all of heaven and earth.
"Repent, for the kingdom of heaven is near."
Matthew 3:3 (NET)