For what partnership have righteousness and iniquity? Or what fellowship has light with darkness? . . . And what agreement has the temple of God with idols? For you are the temple of the living God. As God has said: “I will dwell in them and walk among them. I will be their God, and they shall be My people.” (2 Corinthians 6:14-16)
Coptic Pope Shenouda III offers thoughts on what repentance really is:
If sin is separation from God, then repentance is returning to God. God says: “Return to me and I will return to you” (Malachi 3:8). When the prodigal son repented, he returned to his father (Luke 15:18-20). True repentance is a human longing for the origin from which we were taken. It is the desire of a heart that strayed from God, and finally felt it could go no further away.
For just as sin is conflict with God, so repentance is reconciliation with God. This is what our teacher Saint Paul stated about his apostolic work, saying: “Therefore we are ambassadors for Christ, as though God were pleading by us: we implore you on Christ’s behalf, be reconciled to God” (2 Corinthians 5:20). But repentance is not confined to reconciliation. Through repentance, God returns and dwells in the human heart, transforming it into a heaven. As for the unrepentant, how can God dwell in their hearts while the sin is dwelling therein? As the Bible says, “What communion has light with darkness?” (2 Corinthians 6:14).
…Repentance is resurrection for the spirit, because the death of the spirit is separation from God. As Saint Augustine said: “Repentance is a new pure heart, which God gives to the sinners to love Him with.” It is a divine act performed by God inside the person…
…Not every forsaking of sin is considered repentance. Repentance is the forsaking of sin because of the love of God and the love of righteousness. Other reasons for forsaking sin include fear, embarrassment, inability, preoccupation (with the remainder of love for this sin in the heart), or the consequences of unsuitable situations. These reason are not considered repentance. True repentance is the discarding of sin practically, mentally, and from the heart, which springs out of love for God, His commandments, and His kingdom, and the care of the repentant person for his or her lot in eternity. (The Life of Repentance and Purity, pp. 17-18)