This is basic Luther but wonderful nonetheless.
True, then, are these two sayings: "Good works do not make a good man, but a good man does good works"; "Bad works do not make a bad man, but a bad man does bad works." Thus it is always necessary that the substance or person should be good before any good works can be done, and that good works should follow and proceed from a good person. As Christ says, "A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit" (Matt. vii. 18). Now it is clear that the fruit does not bear the tree, nor does the tree grow on the fruit; but, on the contrary, the trees bear the fruit, and the fruit grows on the trees.
As then trees must exist before their fruit, and as the fruit does not make the tree either good or bad, but on the contrary, a tree of either kind produces fruit of the same kind, so must first the person of the man be good or bad before he can do either a good or a bad work; and his works do not make him bad or good, but he himself makes his works either bad or good.
We may see the same thing in all handicrafts. A bad or good house does not make a bad or good builder, but a good or bad builder makes a good or bad house. And in general no work makes the workman such as it is itself; but the workman makes the work such as he is himself.
Such is the case, too, with the works of men. Such as the man himself is, whether in faith or in unbelief, such is his work: good if it be done in faith; bad if in unbelief. But the converse is not true that, such as the work is, such the man becomes in faith or in unbelief. For as works do not make a believing man, so neither do they make a justified man; but faith, as it makes a man a believer and justified, so also it makes his works good.
Since then works justify no man, but a man must be justified before he can do any good work, it is most evident that it is faith alone which, by the mere mercy of God through Christ, and by means of His word, can worthily and sufficiently justify and save the person; and that a Christian man needs no work, no law, for his salvation; for by faith he is free from all law, and in perfect freedom does gratuitously all that he does, seeking nothing either of profit or of salvation--since by the grace of God he is already saved and rich in all things through his faith--but solely that which is well-pleasing to God.
Taken from "The Freedom of a Christian"