Brake lights are a sign to the drivers following you that you are slowing down or stopping. Street signs tell us where to walk, and where not to walk. Pain can be a sign that something is wrong with your body. A sign points to the presence of something or someone other than itself – an idea, a fact, or a person. Signs can keep us safe. Help us find our way. And even get us back on track when we are lost. Signs are wonderful things. When we can read them.
Why do we seek signs? We need to know. Sometimes we need proof. Maybe we're suspicious of some dodgy claim and want reassurance that we're being told the truth. Trust is hard to come by and a sign will help ground the claim in something more solid than wishful thinking. And even if we're given the requested sign, we have to make the connection between the sign and the truth of the dodgy claim. Does the sign we're given buttress the claim? Or do we need a sign for that too?
Jesus says the only sign we need is his death and resurrection. Believe it happened or don't. No sign given after the fact is going to prove this foundational truth of the faith. There is no sign clear enough, powerful enough to overcome whatever doubts we may have about the truth of the apostolic faith. You meet Christ in the Scriptures, God's creation, or in his Body the church. Or you don't. If you believe, you become the sign of his presence wherever you go, wherever you are. You are his prophet. You are Jonah.