There are several meanings to this word.
1.
The first is the most common. It means sexual intercourse between two people, one or both of whom are already married to another person. It is the willful violation of the marriage contract. It was the only grounds for which a husband could “put away” or divorce his wife.
“The Pharisees also came to him, testing him, and saying to him, Is it lawful for a man to divorce his wife for just any reason? And he answered and said to them, “Have you not read that he who made them at the beginning made them male and female? . . . .whoever divorces his wife, except for sexual immorality, and marries another, commits adultery, and whoever marries her who is divorced, commits adultery.” Mat. 19:1-10 Paul agrees with the “one flesh” teaching of husband and wife, in Eph. 5:22-33 but there is a heavy spiritual meaning woven into this passage too, which will be dealt with next.
2.
The second is not so common. It refers to spiritual adultery, not physical. When Israel turned from the Living God to idols. God said that his people (his Wife) was being unfaithful and committing spiritual adultery when she worshipped idols. ‘Then I saw that for all the causes that backsliding Israel had committed adultery, I had put her away, and given her a certificate of divorce; yet her treacherous sister Judah did not fear, but went and played the harlot also.” Jer. 3:6-10 “Ephraim has hired lovers.” Hos. 8:9 “You are an adulterous wife, who lakes strangers instead of her husband.” Ez. 16:31-34.
A church too can commit adultery, as in Rev. 2:18-26. Thyatira is condemned for allowing a “Jezebel” sprit into her midst. It was Jezebel who killed the prophets of the Lord in the OT. 1 Kings 18:13. It seems peculiar that such a hateful leaching could spring up inside the church, but it did, in the form of the Papacy, wherein the popes proclaimed themselves vicars of Christ, yet slew true Christians in the name of Christ! (This is the Historicist view, which is not accepted by all Christians. The Jezebel spirit may also be interpreted in other ways.)
A generation can be called adulterous. For example, when the scribes and Pharisees came to Jesus asking for a “sign” (and at the same time ignored the many hundreds of astonishing miracles) Jesus said: “An evil and adulterous generation seeks after a sign,” Mat. 12: 38, 39. The Pharisees were such a mixture of Scripture and traditions that they could not discern one from the other. Their traditions blinded them to the truth.
3.
The third is more general. Any immoral thought can be called adulterous. “For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies.” Mat. 15:19,20.
Obviously one does not need to commit any physical act to be an adulterer. Jesus pointed out that if a man lusts after a woman he has “committed adultery with her already in his heart”. It was also the “thoughts of the imagination of the hearts” of the people before the Flood which condemned them to judgment. Externally they may have looked quite ‘respectable’. Likewise the people of Nineveh.
4.
A fourth meaning is “to mix two or more different things together.”
“You shall not let your livestock breed with another kind. You shall not sow your field with mixed seed. Nor shall a garment of mixed woolen and linen come upon you.” Lev.19:19.
“Do not love the world or the things in the world. If anyone loves the world, the love of the Father is not in him.” 1John 2:15-17. The “world” is defined as the desire of the flesh = a craving for sensual gratification, the desire of the eyes = greedy longings, and the pride of life = an assurance of one’s own resources, and a contentment with the stability of earthly things. Christians are told to separate themselves from worldliness – not from the world as God created it. In this case, if a Christian falls away, and begins to allow the world and sin and carnality and disobedience into his or her life, they can be said to be adulterating their Christian life. They are spiritual adulterers.
Adulterate.
The Thesaurus says “To make impure or inferior by deceptively adding foreign substances. To load, doctor, dope, debase or sophisticate. (It is interesting to note in passing that the word ‘sophisticated’ was not a compliment when it was first used, but has since become on. In the same way the word ‘pragmatist’ has also changed to mean someone keen-minded and positive generally, rather than cold and rather ruthless).
A good life may be spoiled by adultery of a different sort. As Paul said: “A little leaven leavens the whole lump.” 1 Cor.5:6-8. In this context, Paul is speaking about the sinful nature (glorying, pride, boasting, malice, wickedness, insincerity, lies) and its effect on holy Christian living.
Having said all this the subject is still wide open to discussion because every Christian has their own interpretation of what constitutes worldliness. As Paul said, one may eat vegetables and one may eat meat. We are not to judge one another, or try to impose our own standards of morality on other Christians. What may be acceptable to one Christian may be offensive to another. The main rule is for each of us to walk before God with clean hands and a good attitude, and then to be careful that our own freedom does not cause another Christian to stumble.