Luther, Preface to The Epistle to the Romans:

Faith is not that human notion and dream that some hold for faith. Because they see that no betterment of life and no good works follow it, and yet they can hear and say much about faith, they fall into error and say, “Faith is not enough; one must do works in order to be righteous and be saved.” This is one reason that when they hear the gospel they fall-to and make for themselves, by their own powers, an idea in their hearts which says, “I believe.” This they hold for true faith. But it is a human imagination and idea that never reaches the depths of the heart, and so nothing comes of it and no betterment follows it.

Faith, however, is a divine work in us. It changes us and makes us to be born anew of God; it kills the old Adam and makes altogether different men, in heart and spirit and mind and powers, and brings with it the Holy Ghost. Oh, it is a living, busy, mighty thing, this faith; and so it is impossible for it not to do good works incessantly. It does not ask whether there are any good works to do, but before the question rises; it has already done them, and is always at the doing of them. He who does not these works is a faithless man. He gropes and looks about after faith and good works, and knows neither what faith is nor what good works are, though he talks and talks, with many words, about faith and good works.

Faith is a living, daring confidence on God’s grace, so sure and certain that a man would stake his life on it a thousand times. This confidence in God’s grace and knowledge of it makes men glad and bold and happy in dealing with God and all His creatures; and this is the work of the Holy Ghost in faith. Hence a man is ready and glad, without compulsion, to do good to everyone, to serve everyone, to suffer everything, in love and praise to God, who has shown him this grace; and thus it is impossible to separate works from faith, quite as impossible as to separate heat and light from fire. Beware, therefore, of your own false notions and of the idle talkers, who would be wise enough to make decisions about faith and good works, and yet are the greatest fools. Pray God to work faith in you; else you will remain forever without faith, whatever you think or do.