Luther produced a torrent of occasional writing, sermons, treatises, commentaries and pamphlets, and no ordered account of what he taught. Melanchthon supplied one. The Loci Communes of 1521, written when he was twenty four, takes the central topics of theology in sequence, sin, law, grace, faith, and works them through in a plain humanist Latin, and it is the first systematic theology the Reformation produced. Luther said of it, with characteristic overstatement, that it deserved a place in the canon.
He was a philologist by training, appointed to teach Greek at Wittenberg at twenty one on the recommendation of his great-uncle Reuchlin, and the training shows everywhere in his work: he wanted texts read accurately and arguments set out in order, and he thought both were teachable.
That instinct produced the other half of his output. He wrote grammars, rhetorics and textbooks, drew up curricula, and reorganised schools and universities across Protestant Germany, to the point that he was called the teacher of Germany. He also drafted the Augsburg Confession of 1530, presented to the emperor as a statement of what the reforming party actually held, deliberately moderate in tone and framed to show continuity with the ancient church. It remains the primary confessional document of Lutheran churches worldwide, and it was written by a layman who never took ordination.