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."
Therefore the Prophet not only calls him a corner-stone, which connects the whole edifice, but also a stone of trial, according to which the building is to be measured and regulated; and farther, he calls him a solid foundation, which sustains the whole edifice. He is thus, then, a corner-stone, that he might be the rule of the building, as well as the only foundation. But Peter took from the words of the Prophet what was especially suitable to his argument, even that he was a chosen stone, and in the highest degree valuable and excellent, and also that on him we ought to build. This honor is ascribed to Christ, that how much soever he may be despised by the world, he may not be despised by us; for by God he is regarded as very precious. But when he calls him a corner-stone, he intimates that those have no concern for their salvation who do not recumb on Christ. What some have refined on the word “corner,” as though it meant that Christ joins together Jews and Gentiles, as two distinct walls, is not well founded. Let us, then, be content with a simple explanation, that he is so called, because the weight of the building rests on him. 