I like the distinction Rutherford makes:
The private forgiving is a duty of charity commanded in the Law of Nature to all, even out of Church-state, and obligeth the Excommunicate, who, though they be case out of the Church, are not exempted from the Law, that bindeth all, Mat. 6.12, 14, 15; Mat. 4.44.46. but the Church-forgiving is an act of obedience to a positive Church-Law of Christ, 2. private Christians are to forgive their Enemies whether they repent, or not, even as Christ forgave those who crucified Him, Col. 3.13. Luk. 23.34. and when the party repenteth not, this forgiveness is not ratified in Heaven, yet are we obliged to forgive, and to commit vengeance to God; but the authoritative forgiveness is a thing that the Church, is not obliged unto, absolutely; nor may they, or can they forgive, except the Offender repent: and if they see that he repenteth not, they cannot lawfully forgive; but, being in God’s room, must take vengeance on all disobedience, and their retaining of sin and forgiving, is valid in Heaven, because they are in God’s place. Now any forgiving or retaining of sin but these two, together with God’s forgiving and retaining, we know not. But Peter’s forgiving his offending brother seventy times seven times, is common to all private Christians, even out of Church-state, and so the instance given is not to the purpose, 6. To these only are the Keys given, who having Paul’s pastoral spirit, may convene and deliver to Satan, but the Church of believers without Officers, not having Paul’s pastoral spirit which is a spirit official, and authoritative to preach, excommunicate, and administrate the seals of the Covenant, may not convene and do this.
- Samuel Rutherford, The Due Right of Presbyteries (Peaceable Plea)