The Greek neuter noun hagion (ἅγιον) is the adjectival form of hagios (holy, G40) used substantively to mean the holy place, the sanctuary, the sacred space. The plural hagia (τὰ ἅγια) refers specifically to the holy compartments of the Tabernacle and Temple — the Holy Place and the Most Holy Place. Hebrews uses this term extensively to describe both the earthly sanctuary (Hebrews 9:1–10) and the heavenly sanctuary into which Christ has entered as High Priest (Hebrews 9:12, 24; 10:19).
The hagion — the sanctuary — was the architectural statement that God dwells among His people. The Holy of Holies (hagia hagiōn, the "most holy place") was where God's presence was enthroned above the Ark of the Covenant, accessible only once a year on Yom Kippur by the High Priest. Hebrews' breathtaking argument is that the earthly sanctuary was only a "copy and shadow" (Hebrews 8:5) of the true, heavenly hagion into which Christ entered with His own blood (Hebrews 9:12). The tearing of the Temple veil at Christ's death (Matthew 27:51) symbolized the opening of the hagion to all believers: the way into the presence of God is now open. Every believer is a walking temple — hagia of the Spirit (1 Corinthians 6:19).