oneness: the state of being unified into a single whole
This is an unusual word for the New Testament. It is a formal, learned term borrowed from Greek philosophy and public life, not from the Old Testament, where the idea of oneness is carried by ordinary words for the number one. It appears only twice in the whole New Testament, both within a few verses of Ephesians 4, where it names the church's unity.
Its two uses there pull in two directions, and that tension is the heart of the word. In Ephesians 4:3 the oneness is something God's Spirit has already made, which believers are told to guard and keep. In Ephesians 4:13 the oneness is a goal the whole church is still growing toward, a maturity it has not yet reached. So the single idea of oneness shows up as both a gift to protect and a target to attain.
Scholars debate whether these are simply two angles on one and the same unity, or two genuinely different unities (the Spirit-given oneness of 4:3 versus the shared faith and understanding of 4:13). Either way, the underlying concept is the same: a real coming-together into one, as opposed to a forced sameness or mere agreement of opinion.
After the New Testament this word took on a life of its own. Within a generation, Ignatius of Antioch made it a favorite term for the unity of the church gathered around its bishop and shared worship, a usage that shaped how later Christians spoke of the one church.