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 (compare Isa. 22:22).

The word is used figuratively of power or authority or office
(Isa. 22:22; Rev. 3:7; Rev. 1:8; compare 9:1; 20:1; compare also
Matt. 16:19; 18:18). The "key of knowledge" (Luke 11:52; compare
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.