Luke refers to the habit we all have of asking God for a sign. We want confirmation of what is happening and are we on the right track. Jesus tells us the only sign we shall have is the sign of Jonah, when Jonah spent three days in the belly of the whale before the whale vomited him out on the shore. This refers to the resurrection of Jesus in the third day. In other words, Jesus is saying....I shall die and rise again, you have belief that I have done this --- have faith in me who can do such an action. He then asks us to simply go on in life sure of his power. But...we are human and we still ask for signs! However, we keep trying, telling the Lord that we beleive and we will trust.