Ephesians 2:19-22: "Now, therefore, you are no longer strangers and foreigners, but fellow citizens with the saints and members of the household of God, having been built on the foundation of the apostles and prophets, Jesus Christ Himself being the chief cornerstone, in whom the whole building, being fitted together, grows into a holy temple in the Lord, in whom you also are being built together for a dwelling place of God in the Spirit."
The apostles and prophets were the foundation of the early church because they pointed to and witnessed to Jesus. The early church was established on their preaching. Christ Himself is the rock foundation on which the whole church rests. The "cornerstone" was the first large stone placed at the corner of a building. The builders would line up the rest of the structure to the chief cornerstone. Being fitted together was a process where construction workers would turn huge rocks around until they fit each other perfectly. God fits believers together into the holy temple He is building for Himself.