When the Bible calls people "adulterous," most often it does so in the sense of "idolatrous." That is what James has in mind here. Idolatry. Friendship with the world is idolatry and these people he is preaching to, these who claim to be Christians, are actually doing what OT Israel did so often - bowing down to the world's idols. In so doing they were breaking their covenant with the Lord and committing spiritual adultery.
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