To begin to answer this question, we need to know what adultery is.  Contrary to popular belief, adultery is a consequence of divorce, and not a cause of divorce.  Often, marital unfaithfulness is equated with adultery, when in fact it is simply sexual immorality within marriage, which may lead to divorce and then adultery.

In my study on marriage based on 1 Corinthians 7, I learnt that adultery is the ultimate forsaking and dissolution of the marriage covenant, and the seal of this is remarriage, because remarriage makes reconciliation after separation or divorce impossible.  Because of the abomination of adultery, Paul instructs Believers who divorce to not remarry, but to remain open to reconciliation.  It is worth asking, therefore; would a breach of this Biblical instruction to remain alone (so as not to commit adultery) after the sin of divorce, make such a person unforgivable?

The concept of ‘living in sin’, which is used to describe Christians who are living with sexual immorality and cohabiting, is usually extended to people who have breached this command to not remarry.  By so judging, their second (or third, fourth…) marriage is…

Read more at https://ufuomaee.blog/is-adultery-an-unforgivable-sin/