The 21st chapter of the Revelation to John is, I think, one of the most sublime in the entire Bible. The fearsome and exhausting wars and conflicts are behind us. The terrifying illusions of lack and separation have been vanquished. Everything that is not eternal, not a pure expression of spiritual truth, has been relegated to the lake of fire to be purified and reborn in more positive ways.
And now comes the payoff. "Then I saw a new heaven and a new earth; for the first heaven and the first earth had passed away, and the sea was no more. And I saw the holy city, the new Jerusalem, coming down out of heaven from God ..." (Rev. 21:1-3). A loud voice announces that "the home of God is among mortals. He will dwell with them; they will be his peoples, and God himself will be with them; he will wipe every tear from their eyes. Death will be no more; mourning and crying and pain will be no more, for the first things have passed away."
All earthly illusions of duality and separation from Source are gone. And now, in the passage you're asking about, "the one who was seated on the throne" speaks for Himself. "'Behold, I make all things new.' And he said, 'Write this, for these words are trustworthy and true.'"
What's important here, I think, is that this “kingdom of heaven” has come to us here, on earth, in the course of our mortal experiences. It's not a question of us ascending somewhere else after death. When we achieve kingdom consciousness, everything will seem new. ("I make all things new" isn't the same as "I make all new things" – it's a promise that the world we live in will be transformed.) And to know that "these words are trustworthy and true" is to know that the promise is absolute, guaranteed, certain to be achieved, no matter how distracted or frightened we may become during the challenges that precede this moment.