The term translated as “for” indicates that with what follows, Peter seeks to provide the reason why he could make the claim that he just made. He had been speaking about Christ as well as the saints as “living stones,” each of which is “being built up as a spiritual house.” He now provides grounds for this claim.
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.”