In Revelation 1:9 John says that he was on the island of Patmos “on account of the word of God and the testimony of Jesus.” He also indicates that he is a fellow participant in their “tribulations.” The Roman historian Tacitus informs us that the Romans used some of the Aegean islands as places of banishment and exile during the 1st century (Annals, 3:68; 4:30; 15:71). Thus the language of the author and the evidence of Tacitus, joined to Christian traditions from the 2nd and 3rd centuries about John’s banishment, support the likelihood that Patmos was a place of exile or political confinement.
In a time when the Asian churches were undergoing persecution, John wrote to them from this island. He addressed each of seven churches by means of a letter of encouragement and warning. The series of letters is followed by the author’s account of the divinely sent vision of impending judgment which “must soon take place” (Rv 22:6). Patmos, then, was the location from which this New Testament writing originated.
Elwell, W. A., & Beitzel, B. J. (1988). In Baker encyclopedia of the Bible (p. 1620). Grand Rapids, MI: Baker Book House.