What is the Purpose of book of Revelation?

Given the situation that brought about the  composition of Revelation and for which the book was written, its purpose and message ought  to be easy to discern. In the first place it exhorts Christians to remain faithful to their Lord despite all the adversities they might have to suffer. That is why it offers reasons for hope to those to those who are disconcerted and besieged by adversities. God is the Lord of Lords and King of the Kings. The Lamb, "the first born of the dead "because of his fidelity to God, is in control of history. By means of hymns and triumphant portrayals John ensures those who remain faithful that just as with the 144,000 and the myriads of others in heaven, they too will participate in the glory ( the wedding feast) of the Lamb in the heavenly Jerusalem.

Second, when he afirms God's justice and final victory John is warning those Christians tempted by syncretism or apostasy that one cannot serve two Lords; the beast and the Lamb, the emperor with his institutions and God.By means of the calamities depicted John categorically affirms that all those who turns their back to on God are headed toward a catastrophic end. His message was therefore uncomplicated for the Christians: assured of the divine sovereignty and justice, be faithful to the Lord! Nobody, nothing will escape God's judgement ! Only those  who are faithful will enjoy the Lamb's marriage and the heavenly Jerusalem .

The centre of attention here, rather than the calamities, is the final triumph; it is Christ and not the emperor of the Roman empire. The overall perspective is focused on present, looking toward the future ( and not on the future, looking back at the present ) : it is to live faithfully today one's commitment to Christ till the end. John's concern is for the Christians, not the Romans.

Thus we see the purpose of the Revelation runs along the lines of liberation and the victory of justice.Fidelity to the Lord presupposes not only having him faithfully in one's daily life, evidenced by ones's "work" . Succinctly put , Revelation is  a book of hope and of eschatological optimism. It is one of the clearest imaginable affirmations of the absolute sovereignty of God and of the Lamb.