Doubt, is the cornerstone of faith. Those who have little doubt have little faith, because we need doubt to grow our faith.
Many great theologians believe this. Many people are intolerant of other religions because they are truly insecure in their own faith because they do not allowed their faith to be challenged. When an individual challenges her faith it will either grow or be shattered to pieces. But if the faith is not challenged, it will be stagnant.