And he carried me away in the Spirit to a GREAT and HIGH MOUNTAIN,
The Revelation of Jesus Christ is a study in contrasts. We know of the Holy Trinity (God, the Father; Jesus, the Son; and the Holy Spirit) from the New Testament. Here in Revelation, we have been introduced to the Unholy Trinity (the Devil; the AntiChrist; and the False Prophet). Even as John was transported to a wilderness (assumed flat and arid land) to see the harlot and her undoing (Revelation 17:3), here we see John transported in the Spirit to a GREAT and HIGH MOUNTAIN.
This is much more spiritual than physical; however, it is real and not imagined for John wrote what he was shown. It reminds us of Psalm 121 where when we lift our eyes to the mountains, we receive help forever. We find an equivalence between the mountains and God. So, too, in this verse, the same equivalency exists.
and showed me THE HOLY CITY, JERUSALEM, coming down out of HEAVEN from GOD,
John has already been given a preliminary view of this HOLY CITY in verse 2. From the vantage point, now, of the HIGH MOUNTAIN, John will get a more detailed view. And he describes that view in the verses that follow. It is an interesting exercise to compare this description to the one Ezekiel gives of 'the temple' (Ezekiel 40-44).
It is THIS HOLY CITY that Abraham was looking for (Hebrews 2:10-13).