The New Jerusalem is a composition of two groups of people—God's Old Testament saints and Christ's New Testament believers. This is signified by the names of the twelve tribes and the names of the twelve apostles (Rev. 21:12b, 14b). The names of the twelve tribes of Israel are inscribed on the twelve gates, the entrance into the holy city. This indicates that the New Jerusalem comprises all the redeemed saints of the Old Testament. The names of the twelve apostles are on the twelve foundations of the holy city. This indicates that the New Jerusalem is also composed of the New Testament saints, represented by the apostles. The Old Testament saints are the entrance, whereas we, the New Testament believers represented by the apostles, are the foundations of the holy city. Without the Old Testament saints, we do not have the entrance, but without us they do not have the foundations. Both the Old and New Testament saints are the composition of the New Jerusalem.
The New Jerusalem is not just a composition but a constitution. Composition is outward, but constitution is inward. Our human body is an example of this. The outward skeleton is a composition of bones, but we also have an inward constitution. The inward constitution of the New Jerusalem is the Triune God, and the outward composition is the redeemed saints and believers. The inward constitution is with God the Father as the substance, signified by the pure gold, with God the Son as the initiation (entrance) through His redemption, signified by the pearls, and with God the Spirit as the building (the wall with its foundations) through His transformation, signified by the precious stones (Rev. 21:18-21).
The New Jerusalem is built of three kinds of precious materials, signifying that she is built with the Triune God. First, the city proper, with its street, is of pure gold (vv. 18, 21). Gold, the symbol of the divine nature of God, signifies the Father as the source, from whom the element for the substantial existence of the city comes.
The twelve gates of the city are twelve pearls. Pearls are produced by oysters in the waters of death. When an oyster is wounded by a grain of sand, it secretes its life-juice around the grain of sand and makes it into a precious pearl. This is a picture of Christ's redeeming and overcoming death and of His life-imparting and producing resurrection with the secretion of His life around us to make us pearls. Christ as the living One came into the death waters, was wounded by us, and secreted His life over us to make us into precious pearls for the building of God's eternal expression.