Q: Answer the question from the given passage. Your answer should be directly extracted from the passage, and it should be a single entity, name, or number, not a sentence.
Passage: Luther's writings circulated widely, reaching France, England, and Italy as early as 1519. Students thronged to Wittenberg to hear Luther speak. He published a short commentary on Galatians and his Work on the Psalms. This early part of Luther's career was one of his most creative and productive. Three of his best-known works were published in 1520: To the Christian Nobility of the German Nation, On the Babylonian Captivity of the Church, and On the Freedom of a Christian. Question: What part of Luther's career was one of his most productive?
A:
early