Albert Barnes Commentary John 2:18

"The Jews therefore answered and said unto him, What sign showest thou unto us, seeing that thou doest these things?" — John 2:18 (ASV)

What sign, etc. What miracle do you work? He assumed the character of a prophet. He was reforming the temple by His authority. It was natural to ask by what authority this was done; and since they had been accustomed to miracles in the lives of Moses, Elijah, and the other prophets, they demanded evidence that He had authority thus to cleanse the house of God.

Seeing that thou doest. Rather, 'by what title or authority you do 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.