Dichotomies

Lately I've been thinking about two dichotomies.

NUMBER ONE
In Sunday School we've been reading and discussing a book called When Heaven Invades Earth. One of the main ideas of the book is that the supernatural move of God in miracles is absolutely essential for a complete Christian life. I've also been studying the Gospels, particularly John and Matthew, and it seems like a major theme in Jesus' earthly ministry was manifesting miraculous "signs" so that people would know He was the Son of God. But then at the very end of the book of John, Jesus turns around 180 degrees and says "Have you believed because you have seen me? Blessed are those who have not seen and yet have believed." So: if signs are necessary for belief, how can the ones that believe without seeing be more blessed than those that believe after seeing? Is faith based on experiencing the supernatural inferior to faith that arrives some other way? And, what else do we have that can generate faith other than experience (if not physical experience, then emotional experiences or at least intellectual stimulation)?

NUMBER TWO
If a healthy tree bears good fruit and cannot bear bad fruit, and if a fig tree can't bear olives/a grapevine can't produce figs/a salt pond can't yield fresh water, how is it that Christians are perfectly able to sin (and most of us do on a daily basis)?

I'm sure these two topics are being hotly debated in a seminary classroom as we speak. Please discuss and elucidate.