The revelation of Jesus Christ which God gave to him to show his bond-servants the things that must happen soon. He made it known when he sent it by his angel to his bond-servant John, who bore witness to the word of God and the testimony of Jesus Christ, to as many things as he saw. Blessed is the one who reads aloud and blessed are the ones who hear the words of the prophecy and keep the things written in it, for the time is near. [Τranslation by David Mills]In addition to calling his document a revelation, John also calls it a prophecy. Prophecy is a word from God spoken by a man whom God had chosen. Hence, we often read of the "word of the Lord" coming to a prophet (1 Sam. 15:10; 1 Kings 21:17; Isa. 38:4; Jer. 1:1-2, etc.). By calling Revelation a prophecy, John is emphasizing that its words are from God.
Although Biblical prophecy is more concerned with speaking the words of God than with revealing the future, there is usually a connection between God's words and the future. Prophets often warn of God's impending judgment on those who disobey and promise future blessings for those who repent and obey. Consequently, the shape of the future depends on whether those who hear ignore his words or keep them in faithful obedience. By calling Revelation a prophecy, John reminds them that they should hear God's warnings and admonitions and that they should keep his commands. To keep means "to persist in obedience" or "to continue to obey orders or commandments." Those who hear and keep the words of John's prophecy will experience God's blessing and favor in the future.
 BDAG, s.v. τηρέω; Louw and Nida, #36.19.