"[I]n the first decades after Luther's death, 'there arose a Lutheran orthodoxy, based on Luther's writings. The reformer was regarded as the timeless prophet of the Christian message. The reception of Luther was concentrated on his written works, which appeared almost as the infallible truth. Church historians even speak of a Protestant scholasticism.'"

I would largely put the blame for that on Melanchthon, who wrote the Augsburg Confession (a piece that did not wholly please Luther) and a Commentary On Paul's Epistle To The Romans. Together, these are the foundations of Lutheran theology.