Jesus heard that they had cast him out; and when He had found him, He said to him, “Do you believe in the Son of God?” He answered and said, “Who is He, Lord, that I may believe in Him?” And Jesus said to him, “You have both seen Him and it is He who is talking with you.” Then he said, “Lord, I believe!” And he worshiped Him. (Jn 9:35–38)
The Lord had given sight to a man blind from his birth; the Lord of nature had removed a defect of nature. Because this blind man had been born for the glory of God, that God’s work might be made manifest in the work of Christ, the Lord did not delay till the man had given evidence of his faith by a confession of it. But though he did not know at the time Who it was that had bestowed the great gift of eyesight, yet afterwards he earned a knowledge of the faith. For it was not the dispelling of his blindness that won him eternal life. And so, when the man was already healed and had suffered ejection from the synagogue, the Lord put to him the question, “Do you believe in the Son of God?” This was to save him from the thought that he had lost everything, being excluded from the synagogue. It gave him the certainty that confession of the true faith had restored him to immortality. When the man, his soul still unenlightened, made answer, “Who is He, Lord, that I may believe in Him?” The Lord’s reply was, “You have both seen Him and it is He who is talking with you.” For His goal was to remove the ignorance of the man whose sight he had restored, and whom He was now enriching with the knowledge of so glorious a faith. Does the Lord demand from this man, as from others, who prayed Him to heal them, a confession of faith as the price of their recovery? Emphatically not. For the blind man could already see when he was thus addressed. The Lord asked the question in order to receive the answer, “Lord, I believe.” The faith which spoke in that answer was to receive not sight, but life.
Hilary of Poitier, On the Trinity 6.48