Key
Frequently mentioned in Scripture. It is called in Hebrew maphteah, i.e., the opener (Judg 3:25); and in the Greek New Testament kleis, from its use in shutting (Matt 16:19; Luke 11:52; Rev 1:18, etc.). Figures of ancient Egyptian keys are frequently found on the monuments, also of Assyrian locks and keys of wood, and of a large size (comp. Isa 22:22).
The word is used figuratively of power or authority or office (Isa 22:22; Rev 3:7; Rev 1:8; comp. Rev 9:1; Rev 20:1; comp. also Matt 16:19; Matt 18:18). The “key of knowledge” (Luke 11:52; comp. Matt 23:13) is the means of attaining the knowledge regarding the kingdom of God. The “power of the keys” is a phrase in general use to denote the extent of ecclesiastical authority.
From Easton’s Bible Dictionary (1897), by Matthew George Easton.
