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."
A chief cornerstone - The principal stone on which the corner of the edifice rests. A stone is selected for this which is large and solid, and, usually, one which is squared, and worked with care; and as such a stone is commonly laid with solemn ceremonies, so, perhaps, in allusion to this, it is here said by God that he would lay this stone at the foundation. The solemnities attending this were those which accompanied the great work of the Redeemer. See the word explained in the notes at Ephesians 2:20.