Revelation 5:9

And they sung a new song
Upon a new occasion and subject, redemption by the blood of the Lamb, and his worthiness to open the sealed book; and in distinction from the old song of Moses and the children of Israel at the Red sea; and this was a most famous and excellent song, an unheard of one, and which none could learn, or sing, but the redeemed of the Lamb:

saying, thou art worthy to take the book, and open the seals thereof.
The Arabic version reads, "thou, O Lamb"; the reasons why they ascribe such fitness, ability, and dignity to him, are as follow:

for thou wast slain:
by men, and for the sins of men; whereby, as he became worthy in his priestly office to take away the sins of his people, and to have all the glory of their salvation, and, in his kingly office, to have all power and authority, and to be exalted above every name, so, in his prophetic office, to have perfect knowledge, as man and Mediator, of all the future events that were to befall his church and people, and to make them known, and fulfil them:

and hast redeemed us to God by thy blood, out of every kindred, and
tongue, and people, and nation;
this shows, that as the four living creatures, and four and twenty elders, were not angels, so they were not representatives of the Jewish church; or the patriarchs and prophets of the Old Testament; not even the apostles of the New; for they were all Jews, and could not be said to be redeemed out of every tongue, nation, and people; and also that not the Jews only were redeemed by Christ, but the Gentiles also, and not all mankind, or every individual of human nature, but some out of all the nations of the earth; for God hath chosen some, both of Jews and Gentiles, and these Christ has redeemed and therefore the Gospel is sent unto all nations, that these among them may be called and saved. The redemption of them supposes them to have been in a state of slavery and bondage, as they are by nature, to sin, Satan, and the law; and signifies a deliverance from such a state, which Christ has obtained, not barely by power, but by price, as the word here used signifies, and may be rendered, "and hast bought us" and the price with which he has bought them in his own "blood", and which is of full and sufficient value, it being not only the blood of a man, of an innocent man, but of one that is God as well as man: and this price was paid "to God", and to his justice, against whom men have sinned, whose law they have broken, and whose justice they have injured and affronted, that he might reconcile them to God, bring them near to him, and that they might serve him in righteousness and true holiness.