The most important function of Christ as the rock is to be the foundation stone of God’s divine building in the universe (Isa. 28:16; 1 Cor. 3:11).
As the foundation of God’s building, Christ must be firm, secure, and unshakable. This requires Him to be a rock that is firm and secure. Only such a foundation stone can enable God’s building to suffer any kind of attack or opposition and not fall (Matt. 7:24-27).
As the divine, massive rock as the foundation of God’s divine building, Christ has sufficient strength to uphold the entire building of God in the universe, that is, His universal church. Christ as the firm foundation stands (2 Tim. 2:19a), and the church, which God has built upon such a firm foundation, cannot be shaken by any person, thing, or matter in the universe. The gates of Hades, Satan’s power of darkness, cannot prevail against the church which is built upon this rock as the firm foundation (Matt. 16:18).
As the living stone and the rock, Christ is not only the foundation stone to uphold God’s building but also the cornerstone (Isa. 28:16; Eph. 2:20b; Acts 4:11; 1 Pet. 2:6-7) to join the different sides, the different sections, of God’s building. God’s universal building is His church, the constituents of which are of different sides, including the Jews and the Gentiles. All these believers of different sides need joining in addition to upholding. Therefore, Christ is not only the foundation stone to uphold the church but also the cornerstone to join the church.
The most important function of Christ as the cornerstone of God’s building is to join together the different sides of God’s building. This is to join the believers, both Jewish and Gentile, to the unique church, which is the holy temple of God in the universe (Eph. 2:18-21), so that the Jewish and Gentile believers may become one Body (2:16; 3:6) as the fullness of God (v. 19) to be the great universal, corporate expression of God.
As the cornerstone of God’s building, Christ is not only for joining the different sides of God’s building but also for supporting the top cover of God’s building. By Christ’s joining, the different sides of God’s building have the strength to stand and become the support of the top covering of God’s building. Without Christ’s joining, the walls of God’s building cannot stand on their own, but through Christ’s joining they can stand and become a strong support for the top covering of God’s building.
If God’s building has only the foundation stone for upholding and the cornerstone for joining but does not have the topstone for covering, it becomes something with open, empty walls and is without any function whatsoever.
As the topstone (Zech. 4:7), Christ is for covering God’s entire building. The church of God enjoys Christ’s covering as the topstone.
As the topstone of God’s building, Christ is not only for covering against forces from the outside but even more for protecting the contents of God’s building. Without the protection of Christ as the topstone, the contents of the church—including the spiritual riches and the saints who enjoy the riches—would have no safeguard and would be damaged and destroyed by the enemy, Satan.