John Cassian, On the Incarnation of the Lord, Against Nestorius III.13: The confession of the blessed Peter receives a testimony to its truth from Christ Himself

But still, as I have made use of the testimony of the chief Apostle, in which he openly confessed the Lord Jesus Christ as God, let us see how He whom he confessed approved of his confession; for of far more value than the Apostle’s words is the fact that God Himself commended his utterance. When then the Apostle said: “Thou art the Christ the Son of the living God,” what was the answer of our Lord and Saviour? “Blessed art thou,” said He, “Simon Barjonah, for flesh and blood hath not revealed it unto thee but the Spirit of My Father which is in heaven.” If you do not like to use the testimony of the Apostle use that of God. For by commending what was said God added His own authority to the Apostle’s utterance, so that although the utterance came from the lips of the Apostle, yet God who approved of it made it His own. “Blessed art thou,” said He, “Simon Barjonah, for flesh and blood hath not revealed it unto thee, but the Spirit of My Father which is in heaven.” Thus in the words of the Apostle you have the testimony of the Holy Spirit and of the Son who was present and of God the Father. What more can you want, or what comes up to this? The Son commended: the Father was present: the Holy Ghost revealed. The utterance of the Apostle thus gives the testimony of the entire Godhead: for this utterance must necessarily have the authority of Him from whose prompting it proceeds. “Blessed then art thou,” said He, “Simon Barjonah, for flesh and blood hath not revealed it unto thee, but the Spirit of My Father which is in heaven.” If then flesh and blood did not reveal this to Peter or inspire him, you must at last see who inspires you. If the Spirit of God taught him who confessed that Christ was God, you see how you are taught by the spirit of the devil if you can deny it.