Philippians 4 (xiii)

Faith is all-powerful and its strength is immeasurable. From this, we can conclude that whoever does not rely on this ‘almighty' faith to overcome that infinitely feeble thing we call the world, then that person does not really have the faith. (...) Faith is the art above all other arts and is a work of the Holy Spirit alone and is known only to God-fearing and true Christians.

The complete Luther - "A Meditation for Each Day of the Year" is available as a biligual (English and German) BREVIARY published by Wartburg Verlag Weimar. It can be purchased in English, world-wide from most internet book shops.