"You shouldn't commit adultery," says the Ten Commandments of the Old Testament. The sentence was said to be set in stone. Supposedly dictated by an invisible god who - in contrast to the thousand other gods of the ancient Orient - was a lonely, chaste, unarmed god. How should he give advice there, one might ask.
Adultery in the bible: who of you is without sin
Marital fidelity is required of all Christians. But the Bible also teaches to be vigilant and to be merciful to sinners.