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."
We must further observe, that the Prophet introduces God as the speaker, for he alone forms and plans his own Church, as it is said in Psalms 78:69, that his hand had founded Sion. He, indeed, employs the labor and ministry of men in building it; but this is not inconsistent with the truth that it is his own work. Christ, then, is the foundation of our salvation, because he has been ordained for this end by the Father. 