What sign ... - What “miracle” dost thou work? He assumed the character of a prophet. He was reforming, by his “authority,” the temple. It was natural to ask by what authority this was done; and as they had been accustomed to miracles in the life of Moses, and Elijah, and the other prophets, so they demanded evidence that he had authority thus to cleanse the house of God.
Seeing that thou doest - Rather “by what title or authority thou doest these things.” Our translation is ambiguous. They wished to know “by what miracle” he had shown, or could show, his right to do those things.