The stone which the builders refused is become the head stone of the corner.  Psalm 118:22

In “olden days” the corner stone was the principal stone placed at the corner of a structure, i.e. at the intersecting angle where two walls of a building come together. In Biblical times, buildings were often made of cut, squared stone. By uniting two intersecting walls, a cornerstone helped align the whole building and tie it together. The cornerstone then was first situated on the foundation, and then all of the other stones were aligned to it. There would need to be one at each corner of the building to provide a firm support, but usually one was larger and more carefully made than the others.

Some scholars think that this "cornerstone" is the keystone in an arch. They suggest that it is the stone which completes the arch and holds the rest of the building together. Our Lord certainly meets this description. He is the topmost stone in the arch, and without Him there would be no strength or cohesion to the building.

A third view is that the stone is the capstone in a pyramid, occupying the highest place in the structure. It is the only stone of that shape in the structure. Its shape determines the shape of the entire pyramid. It is the last stone to be put in place. So Christ is the Capstone of the church, the truly unique Stone. The church gets its character from Him.

This verse, Psalm 118:22,  is quoted five times in the New Testament, and all the references make it clear that the “stone” is the Lord Jesus. The stone, which the builders rejected, has become the head of the corner. When He came into the world He was rejected by His own people—"He came unto His own, and His own received Him not" (John 1:11).

When the Lord came, the Pharisees cast Him away as One that was unwanted. He was not the kind of Messiah they were expecting. They expected Jehovah’s intervention in quite another way, and were looking for Him high up among the great ones of earth. But it is in the church that the Lord Jesus is pre-eminent, being the foundation, the head and the cornerstone. He occupies the same place in the church as the foundation and corner stones do in a building. All the living stones in this spiritual building are built in relation to Jesus Christ the corner stone. One day, however, Israel will receive Him; they will see the error of their ways and say in the words of Psalm 118:22, “the stone which the builders refused is become the head of the corner”.