Easton’s Bible Dictionary
Key: frequently mentioned in Scripture. It is called in Hebrew maphteah, i.e., the opener (Judges 3:25); and in the Greek New Testament kleis, from its use in shutting (Matthew 16:19; Luke 11:52; Revelation 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. Isaiah 22:22).
The word is used figuratively of power or authority or office (Isaiah 22:22; Revelation 3:7; 1:8; comp. 9:1; 20:1; comp. also Matthew 16:19; 18:18). The "key of knowledge" (Luke 11:52; comp. Matthew 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.