Jamieson, Fausset and Brown on Revelation 5:9

9. sung--Greek, "sing": it is their blessed occupation continually. The theme of redemption is ever new, ever suggesting fresh thoughts of praise, embodied in the "new song."

us to God--So manuscript B, Coptic, Vulgate, and Cyprian. But A omits "us": and Aleph reads instead, "to our God."

out of--the present election-church gathered out of the world, as distinguished from the peoples gathered to Christ as the subjects, not of an election, but of a general and world-wide conversion of all nations.

kindred ... tongue ... people ... nation--The number four marks world-wide extension: the four quarters of the world. For "kindred," translate as Greek, "tribe." This term and "people" are usually restricted to Israel: "tongue and nation" to the Gentiles (Re 7:9; 11:9; 13:7, the oldest reading; Re 14:6). Thus there is here marked the election-Church gathered from Jews and Gentiles. In Re 10:11, for "tribes," we find among the four terms "kings"; in Re 17:15, "multitudes."