Revelation 1:20 The mystery of the seven stars which thou sawest in my right hand, and the seven golden candlesticks. The seven stars are the angels of the seven churches: and the seven candlesticks which thou sawest are the seven churches.

The word “mystery” denotes the inner meaning of a symbolical vision”. It speaks of something secret or hidden or is used here in a derived sense of something that contains a hidden or symbolical meaning and something that can be interpreted. [Beckwith]

This word was first used in the New Testament by our Lord Jesus in Matthew 13:11 (cf. Mark 4:11; Luke 8:10) when He answered why He spoke in parables, “Because it is given unto you to know the mysteries of the kingdom of heaven, but to them it is not given.” When Jesus gave a parable, He will also explain its meaning to His disciples.

In the Epistles, it was first used in Romans 11:25 to speak about the mystery of Israel’s blindness in not receiving Jesus as their Messiah for till the last Gentile is saved, “For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in.”