Because it is contained in Scripture, "Behold, I lay in Zion a chief cornerstone, chosen, and precious: He who believes in him will not be disappointed."
The stone which the builders rejected, the same is made the head of the corner ...  It should be pointed out that this famous line is founded upon an actual event.  In the building of Solomon's temple, the first stone that came down from the quarry was very remarkably shaped, having been marked and cut at the quarry.  The builders of the temple did not know what to do with it, and it was dragged to a place apart and became finally hidden by debris and rubbish.  "It was afterward found to be that on which the completeness of the structure depended, the chief corner stone where the two walls met and were bonded together."[26]