A.T. Robertson Commentary Revelation 7:15

"Therefore are they before the throne of God; and they serve him day and night in his temple: and he that sitteth on the throne shall spread his tabernacle over them." — Revelation 7:15 (ASV)

Therefore (δια τουτο). Because of the washing described in verse 14.

They serve him (λατρευουσιν αυτω). Dative case with λατρευω (present active indicative, old verb, originally to serve for hire λατρον, then service in general, then religious service to God, Matthew 4:10, then in particular ritual worship of the priests, Hebrews 8:5). All the redeemed are priests (Revelation 16:5,10) in the heavenly temple (6:9) as here. But this service is that of spiritual worship, not of external rites (Romans 12:1).

Day and night (ημερας κα νυκτος). Genitive of time, "by day and night," as in 4:8 of the praise of the four living creatures.

Shall spread his tabernacle over them (σκηνωσε επ' αυτους). Future (change of tense from present in λατρευουσιν) active of σκηνοω, old verb from σκηνος (tent, tabernacle), used in Joh 1:14 of the earthly life of Christ, elsewhere in N.T. only in Rev. (7:14; 12:12; 13:6; 21:3). In 12:12; 13:6 of those who dwell in tents, here of God spreading his tent "over" (επ' αυτους) the redeemed in heaven, in 21:3 of God tabernacling "with" (μετ' αυτων) the redeemed, in both instances a picture of sacred fellowship, and "the further idea of God's Presence as a protection from all fear of evil" (Swete) like the overshadowing of Israel by the Shekinah and a possible allusion also to the tents (σκηνα) of the feast of tabernacles and to the tent of meeting where God met Moses (Exodus 33:7–11).