Revelation 15:5

Observe, I. How these angels appeared - coming out of heaven to execute their commission: The temple of the tabernacle of the testimony in heaven was opened, Rev 15:5. Here is an allusion to the holiest of all the tabernacle and temple, where was the mercy-seat, covering the ark of the testimony, where the high priest made intercession, and God communed with his people, and heard their prayers. Now by this, as it is here mentioned, we may understand, 1. That, in the judgments God was now about to execute upon the antichristian interest, he was fulfilling the prophecies and promises of his word and covenant, which were there always before him, and of which he was ever mindful. 2. That in this work he was answering the prayers of the people, which were offered to him by their great high priest. 3. That he was herein avenging the quarrel of his own Son, and our Saviour Jesus Christ, whose offices and authority had been usurped, his name dishonoured, and the great designs of his death opposed, by antichrist and his adherents. 4. That he was opening a wider door of liberty for his people to worship him in numerous solemn assemblies, without the fear of their enemies. II. How they were equipped and prepared for their work. Observe, 1. Their array: They were clothed with pure and white linen, and had their breasts girded with golden girdles, Rev 15:6.

— Matthew Henry's Concise Commentary (public domain)

So Rev 11:19; compare Rev 16:17. "The tabernacle of the testimony" appropriately here comes to view, where God's faithfulness in avenging His people with judgments on their foes is about to be set forth. We need to get a glimpse within the Holy place to "understand" the secret spring and the end of God's righteous dealings. behold--omitted by A, B, C, Syriac, and ANDREAS. It is supported only by Vulgate, Coptic, and PRIMASIUS, but no manuscript.

— Jamieson-Fausset-Brown Commentary (public domain)