1 We must now look into and treat of that vision which the Lord showed on the mount. For it is this of which He had said, “Verily I say unto you, there be some standing here which shall not taste of death till they see the Son of Man in His Kingdom.” Then began the passage which has just been read. “When He had said this, after six days He took three disciples, Peter, and James, and John, and went up into a mountain.” These three were those “some,” of whom He had said, “There be some here which shall not taste of death, till they see the Son of Man in His kingdom.”
There is no small difficulty here. For that mount was not the whole extent of His kingdom. What is a mountain to Him who possesses the heavens? Which we not only read He does, but in some sort see it with the eyes of the heart. He calls that His kingdom, which in many places He calls the “kingdom of heaven.” Now the kingdom of heaven is the kingdom of the saints. “For the heavens declare the glory of God.” And of these heavens it is immediately said in the Psalm, “There is no speech nor language where their voice is not heard.
Their sound is gone out through all the earth, and their words unto the end of the world.” Whose words, but of the heavens? And of the Apostles, and all faithful preachers of the word of God. These heavens therefore shall reign together with Him who made the heavens. Now consider what was done, that this might be made manifest.
Source: Sermons on Selected Lessons of the New Testament (New Advent)