Poll Question: On Faith

If you are a Christian, what is the most important facet of your faith? i.e What is the one thing that causes you to believe? If you are not a Christian, what is the most important thing that causes you not to believe (in God, the truth of the Bible, Jesus as the Son of God)? What question is not satisfactorily answered?

I'd love to know your answers, so leave them in the comments. Let the civilized discussion begin!