Hilary of Poitiers
Catena Aurea · Matthäusevangelium 16
When Jesus came into the coasts of Cæsarea Philippi, he asked his disciples, saying, Whom do men say that I the Son of man am? And they said, Some say that thou art John the Baptist; some, Elias; and others, Jeremias, or one of the prophets. He saith unto them, But whom say ye that I am? And Simon Peter answered and said, Thou art the Christ, the Son of the living God. And Jesus answered and said unto him, Blessed art thou, Simon Barjona: for flesh and blood hath not revealed it unto thee, but my Father which is in heaven. And I say also unto thee, That thou art Peter, and upon this rock I will build my Church: and the gates of hell shall not prevail against it. And I will give unto thee the keys of the kingdom of heaven: and whatsoever thou shalt bind on earth shall be bound in heaven: and whatsoever thou shalt loose on earth shall be loosed in heaven.
Otherwise; He is blessed, because to have looked and to have seen beyond human sight is matter of praise, not beholding that which is of flesh and blood, but seeing the Son of God by the revelation of the heavenly Father; and he was held worthy to be the first to acknowledge the divinity which was in Christ.