Adam Clarke on John 7:31

Will he do more miracles - It was the belief of the Jews, and they founded it upon Isa 35:5, that, when the Messiah came, he would do all kinds of miracles; and, in order that they might have the fullest proof of the Divine mission of Christ, it had pleased God to cause miracles to cease for between four and five hundred years, and that John the Baptist himself had not wrought any. His miracles, therefore, were a full proof of his Divine mission.