The pronoun “I” is clearly John. The Lord used John as his means to inform/instruct his people of the new dispensation concerning the things that must take place in the window of time between Jesus’ ascension and his return (Revelation 1:1; Revelation 4:1). He hears the present voice while he is in “a wilderness” (Revelation 17:3).
4 Then I heard another voice from heaven saying, “Come out of her, my people, lest you take part in her sins, lest you share in her plagues;