The old testament
14 You shall not commit adultery. Exodus 20:14, NET©
In the mosaic law, when someone that was married, fornicated with a person outside of the marriage covenant. It was a breach of the marriage, and was called adultery. Adultery is to break the marriage union, and is a serious sin.
In the new testament
In the new testament, the understanding of adultery is sharpened once again.
Adultery at heart – Matthew 5:27-28, NET©
27 You have heard that it was said, “Do not commit adultery.”
28 But I say to you that whoever looks at a woman to desire her has already committed adultery with her in his heart.
The bible says that all our actions begins in with intentions in our hearts. Begins with thoughts that is then expressed in actions. Looking at a married woman with sexual desires is adultery, since it has already taken place in the mind. Similarly, it is fornication to look on an unmarried woman with sexual desires, when it begins at heart.
This commandment also goes in reverse. If a woman looks at a man with sexual desires, it is also adultery or fornication at heart, and is considered a sin.
Divorce – Matthew 5:31-32, NET©
31 It was said, “Whoever divorces his wife must give her a legal document.”
32 But I say to you that everyone who divorces his wife, except for immorality, makes her commit adultery, and whoever marries a divorced woman commits adultery.
We also get an updated view on divorce. In the old testament, they could to divorce and remarry. Jesus says in the new testament that someone who separates for any reason other than sexual sin is guilty of adultery.
Are you an adulterer?
Are you a fornicator or adulterer? All sin begins in the heart! Have you had sexual desire at heart about another person, that is not your spouse? In that case, you are a fornicator or an adulterer. You need to repent of your sin, and ask God for forgiveness and receive Jesus as Savior and Lord.