Then [Jesus] looked at them and said, “What then is this that is written: ‘The stone which the builders rejected has become the chief cornerstone’?” Luke 20:17
In the ancient world, when a new stone building was to be constructed, the first stone set in place was the cornerstone. Great care was taken with the carving of the cornerstone since it determined the direction of the walls that were built out from it. The cornerstone was the most important stone in the building.
To the religious leaders of His day, Jesus intimated that He was the “chief cornerstone.” But of what? To what building was Jesus referring when He declared Himself to be the chief cornerstone? He didn’t say, but the apostle Paul did in Ephesians 2:19-22. Jesus initiated the building of what is becoming “a holy temple in the Lord”—that is, the Church of all true believers, also called “the household of God.” Jesus is the chief cornerstone, “the apostles and prophets” are the foundation, and we are the “living stones” (1 Peter 2:5) God is using to build the temple in which His presence dwells.
Everything depends on Jesus; He is the cornerstone that provides direction and structure to the Church. Our faith begins and ends with Him.
Christ is our temple, in whom by faith all believers meet. Matthew Henry
A Diagnosis of the Christ-Rejecters (Luke 20:19-26)