Adam Clarke on Revelation 5:9

A new song - Composed on the matters and blessings of the Gospel, which was just now opened on earth. But new song may signify a most excellent song; and by this the Gospel and its blessings are probably signified. The Gospel is called a new song, Psa 96:1. And perhaps there is an allusion in the harps here to Psa 144:9 : I will sing a New Song unto thee, O God: upon a Psaltery, and an Instrument of Ten Strings, etc. The same form of speech is found, Isa 42:10 : Sing unto the Lord a New Song, etc.; and there the prophet seems to have the Gospel dispensation particularly in view.

Thou - hast redeemed us to God - out of every - nation - It appears, therefore, that the living creatures and the elders represent the aggregate of the followers of God; or the Christian Church in all nations, and among all kinds of people, and perhaps through the whole compass of time: and all these are said to be redeemed by Christ's blood, plainly showing that his life was a sacrificial offering for the sins of mankind.