Question: Can the sin of adultery be forgiven?
Answer: Moichos (Greek word for "adultery") is defined as, "...unlawful intercourse with the spouse of another (Expository Dictionary of New Testament Words, Vol. 1, pg. 32). Fornication (porneia) is illicit sexual intercourse (Ibid, Vol. 2, pg. 125). Adultery is a specific for the more generic fornication. Sometimes these two words are used interchangeably (cf. Matt. 5: 32; 19: 9).
Adultery is a terrible sin. All sin is horrible (Rom. 6: 23), but some sins are more injurious to innocent people. The innocent husband or wife is severely traumatized by an unfaithful mate. Children suffer because of the marital problems caused by adultery. Society is falling apart, largely because of the deterioration of the family - adultery is often the cause.
Adultery can be forgiven. There are two areas to be explored in answering our question. There is the mate who has been sinned against and there is God. Adultery is a sin against one's mate (Mk. 10: 11). If one is penitent, the sinned against mate is to forgive them. However, I am persuaded the wronged mate has the right to put away, especially in certain circumstances. God will forgive the penitent adulterer. God forgave the Corinthians, some of whom had been adulterers (1 Cor. 6: 9-11).
During thirty years of preaching I have often encountered the argument that "if adultery can be forgiven, then unscriptural marriages may be continued." While we without hesitation affirm adultery can be forgiven, we just as certainly affirm such forgiveness does not mean adulterous marriages are allowed to continue. Jesus said they commit adultery (Matt. 5: 32, 19: 9). "Commit adultery" is from the Greek moichatai. This word is third person, singular, present tense, and in the indicative mood (The Analytical Greek Lexicon, pg. 272). In other words, they continue to commit adultery as long as they actively remain in the relationship.