"This generation is an evil generation; it seeks a sign, but no sign shall be given"

Luke 11:29-33 - When the crowds were increasing, he began to say, "This generation is an evil generation; it seeks a sign, but no sign shall be given to it except the sign of Jonah. For as Jonah became a sign to the men of Nin'eveh, so will the Son of man be to this generation. The queen of the South will arise at the judgment with the men of this generation and condemn them; for she came from the ends of the earth to hear the wisdom of Solomon, and behold, something greater than Solomon is here. The men of Nin'eveh will arise at the judgment with this generation and condemn it; for they repented at the preaching of Jonah, and behold, something greater than Jonah is here. "No one after lighting a lamp puts it in a cellar or under a bushel, but on a stand, that those who enter may see the light."

Our Sign is the Cross – He will not grant you another sign, so that he may give holy things to dogs or throw pearls before swine….
He said only the sign of Jonah will be given to them. This means the Passion on the Cross and the Resurrection from the dead. (St. Cyril of Alexandria)
http://www.byzcath.org/