The answer
I guess as Christians, we assume that all Christians know for sure that adultery is a grave sin and we, therefore, take it for granted and do not discuss it openly. Adultery is the most grievous sinful act which has a harmful effect on the spiritual life because it is committed in secret and through mutual consent. Unfortunately modern secular societies do little to inhibit adultery and sexual promiscuity. Only morality founded on religion can effectively curb this sin. The Old and New Testaments both forbid adultery. In the Old Testament, the seventh commandment given to Moses states: "You shall not commit adultery" (Exodus 20:14)."Whoever commits adultery with a woman lacks understanding" (Proverbs 6:32).
"Let your fountain be blessed, and rejoice with the wife of your youth. As a loving deer and a graceful doe, let her breasts satisfy you at all times; and always be enraptured with her love. For why should you, my son, be enraptured by an immoral woman, and be embraced in the arms of a seductress? For the ways of man are before the eyes of the LORD, and He ponders all his paths. His own iniquities entrap the wicked man, and he is caught in the cords of his sin. He shall die for lack of instruction, and in the greatness of his folly he shall go astray" (Proverbs 5:18-23).
In the New Testament we read,
"Let marriage be held in honor among all, and let the marriage bed be undefiled; for God will judge the immoral and the adulterous" (Hebrews 13:4).
"Be sure of this, that no fornicator or unclean man...has any inheritance in the kingdom of Christ and of God" (Ephesians 5:5).
"But now I have written to you not to keep company with anyone named a brother, who is sexually immoral, or covetous, or an idolater, or a reviler, or a drunkard, or an extortioner—not even to eat with such a person...Therefore 'put away from yourselves the evil person'" (1 Corinthians 5:11-13). Here St. Paul goes even further by prohibiting mixing with adulterers.
Our Lord Jesus Christ raises us to a higher level of purity when He warns us, "But I say to you that everyone who looks at a woman lustfully has already committed adultery with her in his heart" (Matthew 5:28). As a general rule, Christianity does not permit divorce. However, it allows it where adultery has been committed by either spouse Christianity considers adultery as the only justification for divorce This shows how despicable and wicked adultery is in Christian teachings, "And I say to you, whoever divorces his wife, except for sexual immorality, and marries another, commits adultery" (Matthew 19:9). Adultery is definitely a sin against God and against the objective of life. David the prophet and king expressed this fact clearly in Psalms 51 when he repented after committing this sin with Bathsheba "For I acknowledge my transgressions, and my sin is always before me. Against You, You only, have I sinned, and done this evil in Your sight" (Psalm 51:3,4).
Answered by His Eminence Metropolitan Youssef (opens in a new tab)