10 And hast made us unto our God kings and priests: and we shall reign on the earth.
[AD 990] Oecumenius on Revelation 5:8-12
All worshiped the Lord after he had taken the scroll, since they already knew the salvation which he was going to effect for human beings and the punishment he was going to bring on the polluted demons.

The elders’ holding harps indicates the harmony and concord of their confession of God, according to Scripture, “Sing praises to our God, sing praises.”

The incense symbolizes the offering of all the nations, for Malachi, speaking in the person of God, says to disobedient Israel, “I will not accept an offering from your hand, for from the rising of the sun to its setting my name has been glorified among the nations, and in every place incense is offered in my name, and a pure sacrifice,” predicting by these [words] the faith of the nations and their bringing of gifts.

And they sing a new song: for the song sung to God incarnate is new, since it had never been invented before the incarnation.

What was the song? You are worthy, he says, to effect this salvation for human beings, you who were slain for us, and with your blood you took possession of many from among those under heaven.

Very correctly he said from every tribe and tongue and people and nation: for he did not acquire possession of everyone (for many died in unbelief), but only those who were worthy of salvation. The prophet also said something similar, “Arise, O God, judge the earth, because you” will place “your inheritance among all the nations,” where he does not simply say “all the nations.”

And he made them kings and priests to our God, and they shall reign on earth. You can indeed understand this literally, for the kings and presidents of the churches are [God’s] faithful people and servants of Christ. But you can also understand kings to be those who control their passions and are not controlled by them. And you can understand priests to be those who present their own persons “as a living sacrifice, holy and acceptable to God,” as Scripture says.

And he says not only the elders but also the incorporeal powers of the angels were singing a triumphant song to Christ. Daniel also predicted their number as now mentioned. The song of the angels attributes seven different honors to Christ, because by the number seven they indicate that it is fitting for Christ to be crowned with ten thousand praises.

[AD 560] Primasius of Hadrumetum on Revelation 5:9-10
Christ wrote a new song by the harmonious truth of both Testaments. This he did when he, remaining the Word which was in the beginning, was born by a new sort of birth through the Virgin. This he did when he as God put on man in a new manner. This he did when he gave himself over to his killers by his own power. This he did when he was wondrously made alive from the dead and when at a time of his own choosing was ascended beyond the heavens. And this new song he bestowed upon all those who belong to the church which he acquired, that it might be sung continually, and that in this [the church] might rest secure as it awaits in hope the coming judge, that “we might walk in the newness of life.” As he said, “If any one wishes to be my disciple, let him deny himself and take up his cross and follow me.”

[AD 614] Andreas of Caesarea on Revelation 5:9-10
Through this passage it is revealed that the elders, both those in the Old Testament and those in the New Testament, were well-pleasing to God, and that on behalf of the whole world they bring forth a [hymn of] thanksgiving to the Lamb of God who was slain and redeemed us. It is a “new song,” which we have been taught to sing, who from every tribe and tongue have been freed from the antiquity of the letter and through the Spirit have received light. He says that these will rule the new earth, which the Lord promised to the humble.

[AD 235] Hippolytus of Rome on Revelation 5:10
And thou shalt receive the kingdom of heaven, thou who, whilst thou didst sojourn in this life, didst know the Celestial King. And thou shalt be a companion of the Deity, and a co-heir with Christ, no longer enslaved by lusts or passions, and never again wasted by disease. For thou hast become God: for whatever sufferings thou didst undergo while being a man, these He gave to thee, because thou wast of mortal mould, but whatever it is consistent with God to impart, these God has promised to bestow upon thee, because thou hast been deified, and begotten unto immortality. This constitutes the import of the proverb, "Know thyself; "i.e., discover God within thyself, for He has formed thee after His own image. For with the knowledge of self is conjoined the being an object of God's knowledge, for thou art called by the Deity Himself.