The Greek equivalent of Hebrew qadosh (H6918), appearing 233 times. Hagios means set apart, consecrated, and morally pure. Used of God, the Spirit (90+ times), and believers ("saints" — hagioi, "holy ones"). The revolutionary NT move is calling ordinary, imperfect believers hagioi.
The OT confined holiness to sacred space, time, and persons. The NT democratizes holiness: every believer is a "holy one," every community a "holy temple" (Ephesians 2:21), all of life a "living sacrifice, holy and pleasing" (Romans 12:1). This is about being set apart — belonging to God.
The threefold hagios in Revelation 4:8 echoes Isaiah 6:3. Paul calling all believers hagioi — including the problematic Corinthians — shows holiness is first a status (set apart by God) before a process (growing in Christlikeness). The "already/not yet" dynamic is crucial.