Isaiah had prophesied that the stone God would lay in Zion would be a “cornerstone” (Isaiah 28:16). By including the term in his quote, Peter expands on his earlier comment that Christ as living stone, though “rejected by men,” was “in the sight of God chosen and precious,” so precious and chosen as to be made the “cornerstone” of the spiritual house God was building. The term “cornerstone” refers in all likelihood to the (large) stone that tied two walls together (and hence prevented each wall from collapsing).
6 For it stands in Scripture: “Behold, I am laying in Zion a stone, a cornerstone chosen and precious, and whoever believes in him will not be put to shame.”