Then one of the elders addressed me, saying: “These dressed in white robes—who are they and where did they come from?” And I said to him, “My lord, you know.” Then he said to me, “These are the ones who have come out of the great tribulation; they have washed their robes and made them white in the blood of the Lamb.” For this reason, they are before the throne of God and serve him day and night in his temple, and the one seated on the the throne will be a shelter over them. No longer will they go hungry, neither will they thirst again; the sun will not beat down upon them, nor any (scorching) heat; for the Lamb at the center of the throne will shepherd them and guide them to springs of living water, and God will wipe away every tear from their eyes.
A couple of quick notes:
a) There is a strong liturgical element in this text beginning earlier in verse 7. The vestments of this martyred group is liturgical in nature. The word for robes is stolē in the Greek. The color “white” also becomes a liturgical color.
b) “Springs of water” in verse 17 appears later in chapter 14 when speaking of God’s control of all creation. The springs is a source of life for sojourners.
c) This post tribulation scenario involves the Lamb providing shepherding for his people. Hence, the heavenly realm still is incomplete until the parousia.