Central to this verse is the absolute reliability of these words
that the Lord
had sent his angel to show his servants what must soon take place.
That speaks to the concept of prophecy, that is, telling beforehand what will take place in time to come. Old Testament prophets such as Isaiah, Jeremiah, Ezekiel, and Daniel did precisely this (and much of what they foretold had come to pass by John’s day). Yet they could not foretell what would be, without inspiration from the Spirit of God. Their spirits (= thoughts) were clearly guided by the Lord God, him who determines what shall be (Isaiah 44:6; Isaiah 46:8–10). John’s point now is that the same God who guided the thoughts and words of the prophets of old has sent his angel to show his servants what must soon take place.
Given that the prophets’ prophecies happened, surely the content of this Revelation is guaranteed to happen too.
6 And he said to me, “These words are trustworthy and true. And the Lord, the God of the spirits of the prophets, has sent his angel to show his servants what must soon take place.”