What are our doubts?

I'm finishing up preparations for this new sermon series I'm starting in June on the Gospel of Luke. In this book Luke is parading in front of us 100s of "eyewitnesses" to affirm our faith. Then I began asking the question what doubts do each of these eyewitnesses address. Because that is the function of a witness, isn't it? There is a doubt about the truthfulness of an event. Then someone presents witnesses to clear up doubts so we can rely on the truthfulness of the event in question. So what are our doubts about faith? I want to collect as many comments as I can so that I can factor them into my further sermon preparation. So let me know: What are some doubts and uncertainties that people have about faith? What are the uncertainties we find ourselves living with as Christians?