19 Here καί (kai) has not been translated because of differences between Greek and English style.
20 On this term σκηνή 1.b.α states, “ἡ σκηνὴ τοῦ μαρτυρίου the Tabernacle or Tent of Testimony (Ex 27:21; 29:4; Lev 1:1; Num 1:1 and oft.…) Ac 7:44; 1 Cl 43:2, 5,” and then continues in section 2 to state, “Rv 15:5 speaks of a ναὸς τῆς σκηνῆς τοῦ μαρτυρίου ἐν τῷ οὐρανῷ. God’s σκ.= dwelling is in heaven 13:6, and will some time be among humans 21:3.”
21 Grk “the temple of the tent of the testimony” (ὁ ναός τῆς σκηνῆς τοῦ μαρτυρίου, ho naos tēs skēnēs tou marturiou). The genitive “of the tent” is probably an appositional genitive and should be rendered as “the temple, which is the tent.” The entire expression, then, would be “the temple which is the tent of testimony,” that is, “the heavenly equivalent of the tent or tabernacle that was with Israel in the wilderness” (G. K. Beale, Revelation [], ). In the OT the expression “tent of the testimony” occurs frequently (130 times in Exodus through Deuteronomy). The “testimony” refers to the ten commandments, i.e., the revelation of the righteous will of God (Exod 16:34; 25:21; 31:18; 32:15; 40:24). It is little wonder that the wrath of God upon an unrighteous, lawbreaking humanity follows in John’s description.

In other versions

After this I looked, and the sanctuary of the tent of witness in heaven was opened,

After this I looked, and I saw in heaven the temple—that is, the tabernacle of the covenant law—and it was opened.

After this I looked, and the heavenly temple—the tabernacle of testimony—was opened.

Then I looked and saw that the Temple in heaven, God’s Tabernacle, was thrown wide open.

And after these things I looked, and the temple, the tent of the testimony in heaven, was opened,

And after that I looked, and, behold, the temple of the tabernacle of the testimony in heaven was opened:

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