Fear and Faith

Today in class we discussed the relationship between certainty and doubt. To have certainty, is to have 100% faith. To have doubt, is to have no faith. To me, the contradict each other, because if you are certain about something you know it will happen. When you start to doubt, you start thinking about "What if?" Once that what if come, you can not perform the best that you can because, you are fretting and limiting yourself.