James wants his readers to see what their love of the world really is: unfaithfulness to God. "Know ye not" assumes they already know this truth but are living as if they did not. He calls them "adulterers and adulteresses" because, as Barnes explains, they had in effect "broken your marriage covenant with God by loving the world more than him." Gill says such people, instead of loving God with all their hearts, "set their affections upon the world, and the things of it." The question does not allow for a middle ground. Matthew Henry's Concise Commentary says plainly, "Worldly-mindedness is enmity to God."

James answers in the same verse: "whosoever therefore will be a friend of the world is the enemy of God" (James 4:4). He does not leave them there. He reminds them that God "giveth more grace" and "giveth grace unto the humble" (James 4:6), and calls them home: "Draw nigh to God, and he will draw nigh to you" (James 4:8).

The Old Testament often pictures turning from God to idols as unfaithfulness in marriage. Barnes says these words are "frequently applied to those who forsake God for idols," pointing to passages such as Hosea 3:1, Isaiah 57:3, and Ezekiel 16 and 23. The Greek words are sharp opposites: philia, friendship or affection, and echthra, enmity or hostility. The KJV reads "adulterers and adulteresses" because the Textus Receptus has both words (moichoi kai moichalides). Other Greek texts have only the feminine, which the ESV renders "You adulterous people."

This question pictures the bond between God and His people as a marriage covenant, which is how Barnes reads the charge of adultery. God asks for His people's whole heart; Gill says they ought to love Him "with all their hearts and souls." That is why divided love is called unfaithfulness. Gill links this verse to Jesus' words: "God and mammon cannot be loved and served by the same persons" (Matthew 6:24).

Barnes is careful to say what "the world" means here: not the physical world God made, but the world as a community set apart from God's people, with its maxims, principles, and pleasures. He concludes that anyone who is "characteristically a friend of the world" cannot be a true Christian. Yet the very next verses are full of grace. The God who calls unfaithfulness by its name also says, "Draw nigh to God, and he will draw nigh to you" (James 4:8).