The story of the Bible is the story of God's presence among a particular people.
God calls Abraham to journey with him. God calls Moses to deliver the people from slavery in Egypt and when Moses protests God's promise is "I will be with you." As the people wander through the wilderness and enter the promised land, God's presence among them is symbolized by the tabernacle they carry with them. God's presence with Israel is symbolized even more permanently by the Temple that is built during King Solomon's reign. Throughout Israel's story, it is God's presence which makes Israel special, which makes them holy.
However, this presence is also a fearful thing, something that had to be respected, revered, and kept separate. Even though God is present with Israel there is a sense in which God must also always be separate. His immediate presence with Israel would be too powerful, too overwhelming, and too holy so it must always be mediated and kept separate in some way. The presence of God with Israel is something like nuclear energy; it is a power unrivaled by any other but it must also be handled with the most extreme caution. In Leviticus 10, two of Aaron's sons are consumed by fire because they do not follow the priestly rules concerning the tabernacle. In 2 Samuel 6, Uzzah is struck dead by God simply for touching the Ark of the Covenant. Repeatedly, the Old Testament law is concerned is with keep the unholy separate from the holy. This concern for separation is especially evident in the construction of the Temple. While the Temple symbolized God's presence with Israel, the Temple itself was separated into inner and outer courts according to varying degrees of holiness. The Holy of Holies, the innermost part of the Temple where the Spirit of God resided, was entered only by the appropriately cleansed priest and even then only once a year. God was present with Israel and that presence made Israel holy and gave them life but it was a presence so holy and powerful that it had to be mediated and kept separate.
It is against this background that we can understand the kind of radical statement that John is making at the end of Revelation where he describes the holy city, the New Jerusalem, the new creation. Central to this vision of the renewed cosmos are the words of Revelation 21:22; "I saw no Temple in it, for the Lord God, the Almighty and the Lamb are its Temple." In other words, in the new creation there is no need for a building or a tent or anything else to mediate God's presence with God's people. God will simply be present.
The verses just before verse 22 make this same point in a different way. In verse 16, we hear that the holy city is equal in length, width, and height. The city is a perfect cube which seems to be a rather strange detail to include until we recognize that the Holy of Holies in the Temple was a perfect cube as well. It is as if John is saying that whereas before the presence of God was confined to the smallest court of the Temple which could only be entered by one person once a year the presence of God now envelopes this entire holy city. The pure, unmediated, unobstructed, uninhibited presence of God with God's people is central to John's vision of the new creation.