Chapter 15, a relatively short chapter, sets the stage for the next phase of judgments which are to unfold in chapter 16. We basically see two things happening in this chapter.
First, there is a brief description of those who gain victory over the Beast by refusing to take his mark, or the number of his name; even on pain of death.
Second, we see the format through which the judgments of God will be carried out concerning those who do give their allegiance to the Beast.
Most will receive his mark, or the number of his name freely, while others will probably do so in order to avoid the severe consequences of not complying {Rev. 13:15-16}. They will literally sell their souls to the Devil.
Verses 1 - 4
"...seven angels having the seven last plagues..."
Verse 1 offers more evidence that John had a full understanding of the visions of the Book of Revelation, before he ever set forth to put them into a physically written format. In this opening verse, it is stated that he saw the seven angels having the seven last plagues; however, these angels are not actually seen as receiving the bowls of the plagues until we reach verse 7. So it principally serves to inform us of the events which are now about to be described. Before John describes this however, he sees a group of saints in heaven, whom he says are those who have gotten victory over the Beast.
"...sea of glass mingled with fire..."
Here we see martyred saints of the great tribulation, who refused to receive the mark of the beast, or take the number of his name (666). They are seen as standing upon a sea of glass, mingled with fire {Dan. 11:33,35}, having harps in their hands.
The fire here is representative of the tribulation {I Pet. 4:12-13}, which they went through, with the sea of glass speaking of the peace (calm) which they obtained after overcoming the Beast, even by their deaths.
Harps in their hands, and the songs which they sing, are offered as praises to God, who has delivered them through the veil of death into everlasting joy and assurance.
