Notice, not walls and bulwarks for salvation, but salvation for walls and bulwarks. Christ is not only the only foundation of our souls, but His salvation comprises the walls and shield against every enemy of His Church.
The apostles preached Jesus Christ alone (1 Cor. 1:17-23; 3:9-11). They were built on Christ. All who now believe are built on Christ by their ministry. They stood firm because Jesus Christ our Lord saved them. In saving them, He made them know that He is all of their salvation. There is no part of salvation that is not Him (Acts 4:12; Col. 3:11). Peter said it this way: “We believe that through the grace of the Lord Jesus Christ we shall be saved, even as they (Gentiles)” (Acts 15:11). “And if by grace, then it is no more of works” (Rom. 11:6).
Christ lives in His people (Gal. 2:20; Heb. 8:11; 2 Cor. 3:3; Eph. 3:17). It is the belief and confession of Christ that evidences that we are built upon Him (Matt. 16:15-16; 1 John 5:1). Our Lord ordained His apostles. And by the enabling grace of the Spirit of God, which was specially given to the apostles, they were enabled to preach Jesus Christ as the chief cornerstone in the temple of God (Acts 4:11-12). Now, through their ministry, we also "are built upon the foundation of the apostles and prophets, Jesus Christ himself being the chief corner stone; In whom all the building fitly framed together groweth unto an holy temple in the Lord" (Eph. 2:20-21). We are built on Christ Himself. He is our salvation (Isa. 12:2; Luke 2:30). His salvation is the walls and bulwarks of the city into which the righteous nation enters. That nation is made up of all elect, redeemed, believing sinners (Philippians 3:3; Gal. 3:26-29; 4:28). Believing Christ, we keep the truth (Acts 15:9; 2 Tim. 2:25; 1 Pet. 1:22).