What sign showest thou to us? When in so large an assembly no man laid hands on Christ, and none of the dealers in cattle or of the money-changers repelled him by violence, we may conclude that they were all stunned and struck with astonishment by the hand of God. And, therefore, if they had not been utterly blinded, this would have been a sufficiently evident miracle, that one man against a great multitude, an unarmed man against strong men, all unknown man against great rulers, attempted so great an achievement. For since they were far stronger, why did they not oppose him, but because their hands were loosened and -- as it were -- broken? Yet they have some ground for putting the question; for it does not belong to every man to change suddenly, if any thing is faulty or displeases him in the temple of God. All are, indeed, at liberty to condemn corruptions; but if a private man put forth his hand to remove them, he will be accused of rashness. As the custom of selling in the temple had been generally received, Christ attempted what was new and uncommon; and therefore they properly call on him to prove that he was sent by God; for they found their argument on this principle, that in public administration it is not lawful to make any change without an undoubted calling and command of God. But they erred on another point, by refusing to admit the calling of Christ, unless he had performed a miracle; for it was not an invariable rule that the Prophets and other ministers of God should perform miracles; and God did not limit himself to this necessity. They do wrong, therefore, in laying down a law to God by demanding a sign. When the Evangelist says that the Jews asked him, he unquestionably means by that term the multitude who were standing there, and, as it were, the whole body of the Church; as if he had said, that it was not the speech of one or two persons, but of the people.

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.

What sign showest thou - See on Matthew 12:38 (note); Matthew 16:1 (note). When Moses came to deliver Israel, he gave signs, or miracles, that he acted under a Divine commission. What miracle dost thou work to show us that thou art vested with similar authority?

(5) Then answered the Jews and said unto him, What (h) sign shewest thou unto us, seeing that thou doest these things?
(5) Against those who so bind God to an ordinary calling, which they themselves most shamefully abuse, that they will not admit of an extraordinary calling, which God confirms from heaven (and they would have it extinguished, although in vain) unless it is sealed with outward and bodily miracles.
(h) With what miracle do you confirm it, that we may see the heavenly power and strength which gives you authority to speak and to act in this manner?

Then answered the Jews, and said unto him,.... They did not lay hands on him, or offer any violence to him; they did not, as the inhabitants of Nazareth did, thrust him out, and lead him to a precipice, to cast him down headlong; nor did they take up stones to stone him, as they afterwards did, when he asserted his deity: and it is surprising, that they did not rise up and destroy him at once, a single man, unarmed, and without assistance, having so highly provoked them; the restraint upon them must be his almighty power: nor do they deny what he suggested, that they had made his Father's house an house of merchandise; nor do they offer to vindicate their profanation of the temple, or object to the purging of it; only demand a proof of his right to do it: and which demand was made, not by the common people, or by the sellers of oxen, sheep, and doves, and the money changers, who were drove out, and had not spirit to rally again; but by the chief priests and elders, the sanhedrim of the nation, who had the care and government of the temple, and under whose authority the above persons acted; and whose gain and worldly interest were promoted hereby, as a like demand was afterwards made by the same persons; see Matthew 21:23;
what sign shewest thou unto us, seeing thou dost these things? they argued, that either he did these things of himself, by his own authority, and then they must be deemed rash and unjustifiable; or he did it by the authority of others: they knew it was not by theirs, who were the great council of the nation, from whom he should have had his instructions and orders, if he acted by human authority; and if he pretended to a divine authority, as they supposed he did, then they insisted upon a sign or miracle to be wrought, to prove that God was his Father, as he suggested; and that he was the proprietor and owner of the temple, and had a right to purge it, as he had done; see 1-Corinthians 1:22.

What sign showest thou unto us, seeing that thou doest these things?--Though the act and the words of Christ, taken together, were sign enough, they were unconvinced: yet they were awed, and though at His very next appearance at Jerusalem they "sought to kill Him" for speaking of "His Father" just as He did now (John 5:18), they, at this early stage, only ask a sign.

What sign? The Jewish officials demand some sign that would demonstrate that he had authority over the temple.

Then answered the Jews - Either some of those whom he had just driven out, or their friends: What sign showest thou? - So they require a miracle, to confirm a miracle!