Chapter 15 reminds us that one of the key characteristics of God is His justice. He would not be God without this. People who are killed for being Christians would never be able to worship God apart from His justice. He is rightly praised in this chapter for bringing justice on the earth.
In Chapter 15, verses 3-4, we are told those who have been victorious over the beast will sing “The Song of Moses,” and “The Song of the Lamb.” These are the same song, and it is found in Deuteronomy 32:1–43. It expresses the perfect joining of God’s love from the old Covenant with the New Covenant, and focuses on praising and glorifying God.