Self.
Faith will never be weak if self be weak, but when self is strong, faith cannot be strong; for "self" is very much like what the gardener calls the "sucker," at the bottom of the tree, which never bears fruit, but only draws away the nourishment from the tree itself. Now, self is that sucker which diverts the nourishment from faith, and you must cut it up, or else your faith will always be "little faith," and you will have difficulty in maintaining any comfort in your soul.