The man asked because he wanted to believe but did not yet know whom to believe in. He had never seen Jesus. He received his sight only after he washed in the pool of Siloam, away from Jesus (John 9:7, 11). Barnes says the man probably did not know that the one speaking to him was the one who had healed him, and that his question "shows that he was disposed to believe, and had a strong desire to see and hear the Son of God." His question is not a dodge. It ends with its purpose: "that I might believe on him."

Jesus answered plainly: "Thou hast both seen him, and it is he that talketh with thee" (John 9:37). The man replied, "Lord, I believe. And he worshipped him" (John 9:38). Barnes calls this "the overflowing expression of gratitude and faith" and says the man "did homage to him as the Messiah and as his gracious benefactor."

The word the KJV translates "Lord" can be a simple polite address. Barnes notes, "This word here, as in many other instances in the New Testament, means 'Sir,'" which is how the ESV renders it. When the man speaks again after Jesus reveals Himself, he says "Lord, I believe" and worships Him (John 9:38).

The man's first sight came at the pool of Siloam, which John explains means "Sent" (John 9:7). So the first face he could have recognized as his healer's was this one, after the trial before the Pharisees.

Here Jesus answers an honest seeker. The Pharisees asked many questions in John 9, but they had already decided, "we know that this man is a sinner" (John 9:24). This man asked one question, and he asked it in order to believe. Jesus answered him directly, the way He told the woman of Samaria, "I that speak unto thee am he" (John 4:26), which Gill also connects to this verse.

Matthew Henry writes, "Our Lord Jesus graciously reveals himself to the man." The man who once could see nothing now sees the Son of God face to face.