‘After these things I saw, and the Temple of the Tabernacle of the Testimony in Heaven was opened, and there came out from the Temple the seven angels that had the seven plagues, arrayed with precious linen (some few authorities read ‘stone' - lithon for linon), pure and bright, and girded about their chests with golden girdles.'
‘After these things' does not mean ‘chronologically following' except from John's point of view (compare Revelation 7:1). It means he saw one thing, and then moved on to the next, but it says nothing about how the two visions fitted in time sequence. Compare how the glorified people of God in Heaven are seen prior to the blowing of the seven trumpets (Revelation 7:9-17).
‘The Temple of the Tabernacle of the Testimony.' Stress is laid here on the fact that this Heavenly Temple is the equivalent of the Tabernacle of the Testimony (Exodus 38:21), which contained the Ark of the Testimony (Exodus 26:33-34; Exodus 30:6; Exodus 30:26) which itself contained the two tables of the Testimony (Exodus 31:18; Exodus 32:15; Exodus 34:29), the covenant of grace which God had made with Israel through Moses, which included the ten commandments (Exodus 25:16; Exodus 25:21-22). Thus God is about to deal with those who have broken His law and to fulfil His promises of protection and deliverance to His people. Compare Revelation 11:19 where the Ark of His Covenant was seen in Heaven. The Ark was in the Holy of Holies, but there is now no Temple on earth.
The seven angels are girded in such a way as to suggest a priestly function. The golden girdle is like that worn by the Son of Man in Revelation 1:13. (The variant reading would make no difference to the sense, but if ‘stone' is read it could refer to something similar to the white stone given to overcomers (Revelation 2:17)). Priests were responsible for guarding the book of the Law (Deuteronomy 17:18), teaching the Law (Malachi 2:7) and acting as judges in some cases (Deuteronomy 17:8-9). They are the messengers (the word also means ‘angels') of the Lord of Hosts (Malachi 2:7). Thus we are to see these angels as acting as priests and messengers of God in condemning, and meting out punishment to those who have broken God's law and refused to repent.