Jamieson, Fausset and Brown on Isaiah 63:19

19. thine ... never--rather, "We are Thine from of old; Thou barest not rule over them" [Barnes]. Lowth translates, "We for long have been as those over whom Thou hast not ruled, who are not called by Thy name"; "for long" thus stands in contrast to "but a little while" (Is 63:18). But the analogy of Is 63:18 makes it likely that the first clause in this verse refers to the Jews, and the second to their foes, as English Version and Barnes translate it. The Jews' foes are aliens who have unjustly intruded into the Lord's heritage.