The biblical interpreter who does not suffer from anti-supernaturalism will forgo attempts to plumb the depths of Johns psyche to establish the purpose of his writing. Instead, he will recognize that John himself had no purpose! But that God, by direct supernatural intervention, simply told John to write. And as a dutiful servant, John did just that!
The purpose of the book is not found in John, but in the plain statements given in the book and by an understanding of the content of the book. Mills provides an excellent summary:
The divine purpose in this book can be seen as threefold. The first purpose is to reveal Christ in His deistic glory. Chapter 1 introduces the risen Christ as possessing all the effulgence of the glory of the Father and also presents Him in a judgmental capacity (the sword of Rev. Rev. 1:16+). The purpose of revealing Christ in this glory is to depict clearly and unmistakably His deity-a fact which was already under question by some heretical sects-and also to present Him in an unmistakable judgmental role. This purpose, stated in chapter Rev. 1:1+, pervades the whole book; . . . The second purpose is explicit in Chapters Rev. 2:1+-Rev. 3:1+, and meets the particular needs and circumstances of particular churches at a particular time. . . . to address the needs of the seven churches, and thereafter to leave a body of admonition, commendation, and promise which would be applicable to the Church Age . . . . The third purpose, achieved in Chapters 4-22, is to confirm the apocalyptic and millennial prophecies of the Old Testament. With the emergence of the mystery age, the Church Age, questions would understandably arise as to whether Gods program had been abandoned, modified or whatever. The purpose of these nineteen chapters is to reaffirm these prophecies, to consolidate and supplement them into a fuller and more chronological record. . . A secondary purpose is to give the Church a focus for the hope that each believer has by giving a preview of the Millennial Kingdom and of the eternal life which follows it. Revelation thus completes the New Testament argument by summarizing and consolidating those Old Testament prophecies of the Messianic Age of Righteousness which were unfulfilled at the end of the apostolic age, and by clarifying that these prophecies were still to be fulfilled at, or just prior to, our Lords second advent, at which time He will set up the earthly kingdom prophesied in the Old Testament (we now know this as the Millennial Kingdom from Revelation Rev. 20:1+). Secondly, Revelation also completes the New Testament presentation of Jesus Christ by displaying Him in His eternal glory, thereby refuting any attempt to leave Jesus as merely human or less than fully God. And thirdly, Revelation addresses the needs of the Church at the end of the apostolic age, thus leaving a picture of a Church as diverse as that which has succeeded it, and, consequently, a body of principles which provide admonition, commendation, and promise that is applicable throughout the Church Age. [emphasis added]1
1 Monty S. Mills, Revelations: An Exegetical Study of the Revelation to John (Dallas, TX: 3E Ministries, 1987), s.v. Introduction.