Adultery is sexual intercourse by a married person with someone who is not their spouse. One of the first ten commandments from God was:
Thou shalt not commit adultery. (Exodus 20:14)
In Old Testament times when a person committed adultery he was put to death:
And the man that committeth adultery with another man's wife, even he that committeth adultery with his neighbour's wife, the adulterer and the adulteress shall surely be put to death. (Leviticus 20:10)
Both Jesus and Paul repeated the warning against adultery in the New Testament:
Thou knowest the commandments, Do not commit adultery... (Mark 10:19)
For this, Thou shalt not commit adultery... (Romans 13:9)
Jesus expanded the meaning of adultery to include evil sexual desires of the heart:
Ye have heard that it was said by them of old time, Thou shalt not commit adultery:
But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart. (Matthew 5:27-28)
Adultery also includes divorcing a mate and remarrying without Biblical cause:
But I say unto you, That whosoever shall put away his wife, saving for the cause of fornication, causeth her to commit adultery; and whosoever shall marry her that is divorced committeth adultery. (Matthew 5:32)
And if a woman shall put away her husband, and be married to another, she committeth adultery. (Mark 10:12)
When a person commits adultery he is sinning against his own soul:
But whoso comitteth adultery with a woman lacketh understanding: he that doeth it destroyeth his own soul. (Proverbs 6:32)
God judges those who commit adultery:
Marriage is honourable in all, and the bed undefiled: but whoremongers and adulterers God will judge. (Hebrews 13:4)
Those who commit adultery do not inherit the Kingdom of God:
Know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with mankind,
Nor thieves, nor covetous, nor drunkards nor revilers, nor extortioners, shall inherit the Kingdom of God. (I Corinthians 6:9)
One of the characteristics by which you can recognize false teachers is by the sin of adultery:
...there shall be false teachers among you...Having eyes full of adultery, and that cannot cease from sin... (II Peter 2:1,14)
The Bible warns:
...the adulteress will hunt for the precious life. (Proverbs 6:26)