Very sanctified and spiritual people can sadly be doubters. Faith and piety don't always go down the street hand in hand. The idea is also around that you have to be a spiritual giant to have great faith. It is all confused. It is the man or woman strong in faith who is the spiritual giant.
Faith is spiritual strength.
Faith produces good works, but good works don't produce faith - just as milk produces butter but butter does not produce milk. However, there's more to be said about that in the next twenty plus chapters.