"I," he answered, "am--'The voice of one crying aloud in the Wilderness--"straighten the way of the Lord"', as the Prophet Isaiah said."

In fulfillment of the words of the Prophet Isaiah, where he says--'Lord, who has believed our teaching? And to whom has the might of the Lord been revealed?'

The reason why they were unable to believe is given by Isaiah elsewhere, in these words--

Isaiah said this, because he saw Christ's glory; and it was of him that he spoke.