Adam Clarke on John 9:38

And he said, Lord, I believe - That is, I believe thou art the Messiah; and, to give the fullest proof of the sincerity of his faith, he fell down before and adored him. Never having seen Jesus before, but simply knowing that a person of that name had opened his eyes, he had only considered him as a holy man and a prophet; but now that he sees and hears him he is convinced of his divinity, and glorifies him as his Savior. We may hear much of Jesus, but can never know his glories and excellencies till he has discovered himself to our hearts by his own Spirit; then we believe on him, trust him with our souls, and trust in him for our salvation. The word κυριε has two meanings: it signifies Lord, or Sovereign Ruler, and Sir, a title of civil respect. In the latter sense it seems evidently used in the 36th verse, because the poor man did not then know that Jesus was the Messiah; in the former sense it is used in this verse - now the healed man knew the quality of his benefactor.