Read Exodus 20:14
14 “You shall not commit adultery. (NIV)
The commandments given by God to order our personal relationships are generally reflective of the character of God himself. God is truthful so we ought not to lie. God is honest so we ought not to be dishonest. This particular commandment reflects God’s faithfulness. He sticks by us according to his promises, so we should stick to others according to the promises we make them.
This command means we ought not to break our promises to our marriage partners to be faithful to them. But we need to note two issues which may be overlooked. One is that being faithful in marriage involves more than just refraining from sex with someone else.
Faithfulness is not just staying out of someone else’s bed. It means being positively faithful within your own marriage. It is not just a negative refraining from other commitments but positively regarding our existing commitments. A dried-up marriage is a breeding ground for adultery.
Which leads to another issue: adultery is not just a problem in itself but the symptom of an existing problem. Not much point in urging someone to give up an affair if we don’t also urge that couple to work on the marriage or the individual to work on his or her character. This is not to excuse adultery but to understand that it doesn’t just spring out of nowhere.
Ultimately, the key to keeping faith in our human relationships is to keep faith in the God who keeps faith with us.