I, Jesus, have sent My angel to testify to you these things in the churches
The Revelation of Jesus Christ (the contents of which are recorded by this book), was given by God to the Son Who then sent and signified it by His angel to John (Rev. Rev. 1:1+). See commentary on Revelation 1:1. Testify is μαρτυρῆσαι [martyrēsai] : testify or witness. See commentary on Revelation 1:2.
Until Jesus returns, we have the prophetic word, such as this very book, to serve as a beacon of hope while we continue in this dark world:
And so we have the prophetic word confirmed, which you do well to heed as a light that shines in a dark place, until the day dawns and the morning star rises in your hearts; knowing this first, that no prophecy of Scripture is of any private interpretation, for prophecy never came by the will of man, but holy men of God spoke as they were moved by the Holy Spirit. (2Pe. 2Pe. 1:19-21)
Yes, the day is not herebut lo, the harbinger of the day, the Morning Star! It shines in the night, but it prophesies the coming sunrise. The assembly (ecclesiathe Church) sees Him in the now far spent night as the Morning Star, recognizes Him, while watching for Him, according to His own Word, in His bright heavenly charactera character which does not wake a sleeping world, but is the delight and joy of those who watch. When the sun arises, He will not be thus known: the earth will never so know Him, bright as the (coming) day will be (Darby).5
2 Frederick William Danker and Walter Bauer, A Greek-English Lexicon of the New Testament and Other Early Christian Literature (Chicago, IL: University of Chicago Press, 2000), 725.
3 Ibid., 62.