The Apology of the Augsburg Confession was written by Philipp Melanchthon during and after the 1530 Diet of Augsburg as a response to the Pontifical Confutation of the Augsburg Confession, Charles V's commissioned official Roman Catholic response to the Lutheran Augsburg Confession of 25 June 1530. It was intended to be a defense of the Augsburg Confession and a refutation of the Confutation. It was signed as a confession of faith by leading Lutheran magnates and clergy at the meeting of the Smalkaldic League in February, 1537, and subsequently included in the German [1580] and Latin [1584] Book of Concord. As the longest document in the Book of Concord it offers the most detailed Lutheran response to the Roman Catholicism of that day as well as an extensive Lutheran exposition of the doctrine of Justification. Source: Wikipedia (en)