10 C's: #7 Adultery part c

You shall not commit adultery Exodus 20:14

The church in Jerusalem wanted the Gentiles to avoid a couple really important behaviors. In Acts 15 (vs. 20, 29) we learn the big ones to avoid were food sacrificed to idols, blood, strangled animal meat, and immoral sex. In Paul's letters to the church in Corinth he assured them eating sacrificial food is fine if your conscience isn't bothering you and no one with a sensitive conscience sees you do it. The other two are never mentioned again. But sex comes up frequently. There are sin lists such as in 1 Timothy 1

8 We know that the law is good if one uses it properly. 9 We also know that law is made not for the righteous but for lawbreakers and rebels, the ungodly and sinful, the unholy and irreligious; for those who kill their fathers or mothers, for murderers, 10 for adulterers and perverts, for slave traders and liars and perjurers--and for whatever else is contrary to the sound doctrine 11 that conforms to the glorious gospel of the blessed God, which he entrusted to me. (NIV)

and the last half of Romans 1. Two hundred years after these verses were penned the church looked down hard on some of these sins if committed after baptism. They figured something begun in the Spirit could be finished in the flesh. So some big sins like adultery, murder and idolatry were unforgivable after baptism. Many other things could be overcome with some hard penance, but not adultery. Kelly writes in Early Christian Doctrines

The most noteworthy advance in the theology of penance in the third century was in connexion with the Church's attitude to certain sins esteemed particularly heinous. In the last decades of the second century adultery, homicide and idolatry (or apostasy) seem to have been treated in practice, if not in theory, as irremissible, even by means of the once-for-all exomologesis [hard core post-baptism penance] described above...Certainly Hippolytus, protesting against Callistus's innovations, and Tertullian in his later Montanist phase took it for granted that it had been the Church's practice to reserve such sins hitherto. Origen supplies confirmatory evidence for the East, explaining that, under the guidance of the Holy Spirit, the good bishop 'forgives whatever sins God forgives, but reserves others which are incurable....' He quotes 1 Sam. 2:25 ('If a man sin against the Lord, who shall intreat for him?), a classic text in discussions about penance, and adds that idolatry, adultery and fornication figure among these sins for which there is no remedy. Cyprian is an important witness, for he shows (a) that, while sexual sins were irremissible at Carthage in his day, there had previously been disputes on the subject: and (b) that idolatry, irremissible in the past, only came to be included among sins capable of forgiveness as a result of the Decian persecution. (217-219)

The early church was wrong about unforgiveness but were right about the gravity of the sin, but anyone who has been betrayed in their marriage could confirm that.