koinos

This is the everyday Greek meaning of the word, the one it carried long before the New Testament. It describes anything held or owned in common: a shared courtyard, joint property, a purse everyone draws from. In the book of Acts it is the word for the way the first believers pooled their possessions so that no one owned things privately.

The same idea stretches from things to what people hold together in faith. Paul greets Titus as a partner in a faith they both share, and Jude writes about a salvation that belongs equally to every believer. In each case the word points to something that is genuinely everyone's at once. This shared, communal sense is the neutral side of the word, and it is the root behind the Greek word for fellowship and partnership. It is worlds away from the second, darker meaning the word picked up among Greek-speaking Jews.

Among Greek-speaking Jews the word took on a meaning it never had in ordinary Greek. Alongside "common" it came to mean "defiled, unhallowed, ritually unfit." The shift seems to have begun during the persecution under Antiochus in the second century before Christ, when faithful Jews needed a word for the pork and pagan food they were being forced to eat. What the surrounding nations ate "in common," with none of Israel's holiness on it, was exactly what made it intolerable, and "common" hardened into "defiled."

By the first century it had become a near-technical term for the boundary of the observant Jewish table. This is the word for the "defiled hands" that Jesus' critics complain about in Mark 7, and the word Peter uses on the rooftop when he protests that he has never eaten anything "defiled or unclean." It is worth watching how it differs from a neighbor word for "unclean" (ἀκάθαρτος): that neighbor names things unclean in themselves, like the forbidden animals, while this word often names something otherwise permitted that has become defiled through contact, mishandling, or passing through the wrong hands.

The word does two kinds of work. In the Gospels, Acts, and Romans it belongs to the world of food, hands, and table fellowship. But in Hebrews and Revelation it turns to a graver, moral sense: treating the covenant blood of Christ as though it were ordinary and unhallowed, or being shut out of the holy city because one is unfit for God's presence. The classical Greek word for this "outside the sacred" idea was a different term (βέβηλος); this word quietly took over its ground in Jewish speech.