Albert Barnes Commentary John 6:69

"And we have believed and know that thou art the Holy One of God." — John 6:69 (ASV)

We are sure. See a similar confession of Peter in Matthew 16:16, and see also the notes on Matthew 16:16.

Peter says we are sure, in the name of all the apostles. Jesus immediately cautions him, as He did on other occasions, not to be too confident, for one of them actually had no such feelings but was a traitor.