Jamieson, Fausset and Brown on Revelation 1:19

19. The oldest manuscripts read, "Write therefore" (inasmuch as I, "the First and Last," have the keys of death, and vouchsafe to thee this vision for the comfort and warning of the Church).

things which are--"the things which thou hast seen" are those narrated in this chapter (compare Re 1:11). "The things which are" imply the present state of things in the churches when John was writing, as represented in the second and third chapters. "The things which shall be hereafter," the things symbolically represented concerning the future history of the fourth through twenty-second chapters. Alford translates, "What things they signify"; but the antithesis of the next clause forbids this, "the things which shall be hereafter," Greek, "which are about to come to pass." The plural (Greek) "are," instead of the usual Greek construction singular, is owing to churches and persons being meant by things" in the clause, "the things which are."