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."
behold, I lay in Sion a chief corner stone, elect, precious; Christ is here called a chief corner stone, as in Ephesians 2:20 because he not only adorns and strengthens the building, but unites the parts, and keeps them together, even all the saints, Jews or Gentiles, in all ages and places, whether in heaven or earth; and he, as such, is chosen of God for that purpose, and is precious both to God and man, on that account; and is a stone, not of men's laying, but of God's laying in his council, covenant, promises, and prophecies, in the mission of him into this world, and in the Gospel ministry; the place where he is laid is in Sion, the Gospel church, of which he is both the foundation and corner stone: and this account is introduced with a "behold", it being something very wonderful, and worthy of attention: to which is added,