Rev. 15:2 And I saw something like a sea of glass mingled with fire, and those who have the victory over the beast, over his image and over his mark and over the number of his name, standing on the sea of glass, having harps of God.
3 They sing the song of Moses, the servant of God, and the song of the Lamb, saying: “Great and marvelous are Your works, Lord God Almighty! Just and true are Your ways, O King of the saints!
4 Who shall not fear You, O Lord, and glorify Your name? For You alone are holy. For all nations shall come and worship before You, For Your judgments have been manifested.”
These, I believe are the tribulation saints, also spoken about in chapter 7. They have victory over the beast, never take the mark, but will be martyred. No worries, life only begins in eternity. There is no death, or seperation from God, to a believer. In fact at this point, all their attention is upon the LORD. They are caught up in worship. What problems? Their focus is upon God. When I get my eyes on the LORD, I discover my problems are very small. In fact, we should hand all them over to the LORD and leave them there.