In verse 9 the Lord told the brother of Jared that he was able to see because of his great faith. At this juncture, the nature of the vision is about to be expanded, and the Lord makes it clear that the basis for this expanded vision will be that same faith. This is the reason for the interrogation. When the Lord asks “believest thou the words which I shall speak?” he is asking the brother of Jared for a declaration of faith. That faith is declared in the brother of Jared’s firm reply: “Yea, Lord, I know that thou speakest the truth, for thou art a God of truth, and canst not lie.”
With the receipt of the correct answer, symbolically indicating the demonstration of faith, the Lord shows himself to the brother of Jared, as requested.