John Wesley on Revelation 15:2

The song was sung while the angels were coming out, with their plagues, who are therefore mentioned both before and after it, verses 1 - 6. Rev 15:1 - 6 And I saw as it were a sea of glass mingled with fire - It was before "clear as crystal," Rev 4:6, but now mingled with fire, which devours the adversaries. And them that gained, or were gaining, the victory over the wild beast - More of whom were yet to come. The mark of the beast, the mark of his name, and the number of his name, seem to mean here nearly the same thing. Standing at the sea of glass - Which was before the throne. Having the harps of God - Given by him, and appropriated to his praise.