This is from Luther's Preface to St. Paul's Epistle to the Romans (1522)

Faith is a divine work in us, which transforms us, gives us a new birth out of God (John 1:13), slays the old Adam, makes us altogether different men in heart, affection, mind, and all powers, and brings with it the Holy Spirit. Oh, it is a living, energetic, active, mighty thing, this faith! It cannot but do good unceasingly. There is no question asked whether good works are to be done, but before the question is asked the works have been done, and there is a continuous doing of them. But any person not doing such works is without faith. He is groping in the dark, looking for faith and good works, and knows neither what faith is nor what good works are, although he indulges in a lot of twaddle and nonsense concerning faith and good works.

Faith is a living, daring confidence in the grace of God, of such assurance that it would risk a thousand deaths. This confidence and knowledge of divine grace makes a person happy, bold, and full of gladness in his relation to God and all creatures. The Holy Spirit is doing this in the believer. Hence it is that a person, without constraint, becomes willing and enthusiastic to do good to everybody, to serve every-body, to suffer all manner of afflictions, from love of God and to the praise of Him who has extended such grace to him. Accordingly, it is impossible to separate works from faith, just as impossible as it is to separate the power to burn and shine from fire. Accordingly, beware of your own false thoughts and of idle talkers, who pretend great wisdom for discerning faith and good works and yet are the greatest fools. Pray God that He may create faith in you; otherwise you will remain eternally without faith, no matter what you try to do or fabricate.