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."
And he says in Sion, because there God’s spiritual temple was to have its beginning. That our faith, therefore, may firmly rest on Christ, we must come to the Law and to the Prophets. For though this stone extends to the extreme parts of the world, it was yet necessary for it to be located first in Sion, for there at that time was the seat of the Church. But it is said to have been then set, when the Father revealed him for the purpose of restoring his Church. In short, we must hold this, that those only rest on Christ, who keep the unity of the Church, for he is not set as a foundation-stone except in Sion. As from Sion the Church went forth, which is now everywhere spread, so also from Sion our faith has derived its beginning, as Isaiah says, 