Lutherans teach that salvation is a free, unearned gift from God, bestowed on humans if only they believe in Him. This was earth-shattering, radical news at a time, and for many it still is today! The Catholic establishment abhorred Luther's teaching of salvation without any "good works". If people do not work to earn salvation, so they assured, this would promote idleness and cause all kinds of societal ills.
Criticism grew quickly. In 1520 Luther responds: Good works are still done by believers. But these Good Works follow salvation and do not cause it. Good Works, Luther assured, are happily done by the already-rescued soul in gratitude - not to earn its salvation. The entire concept of "vocation", of one's profession, of one's "job" as a calling from God, originates here.
Writing to be easily understood by the common man, Luther's fresh and engaging style has lost nothing of its clarity and vigor: we "moderns" readily understand it. And his message is as relevant as ever.