Matthew 16:4 A wicked and adulterous generation seeks after a sign, and no sign shall be given to it except the sign of the prophet Jonah.” And He left them and departed.
When we stop trusting in Christ, give up on our faith and allow the doubts to build up in our heads we leave Christ out of our lives.
Just as these Pharisees and Sadducees were being told the only signs they would see would be what was told them in the past, so too Christ is warning us.
Not trusting in our faith means we are only left with what we have seen in the past; there will be a time when nothing new will encourage us.