Today is known as Reformation Day among Lutherans because October 31, 1517 was the date Martin Luther posted his 95 Theses — a date that historians consider to be the beginning of the Protestant Reformation.
When I was a Lutheran (from my early 20s until my late 30s), I read the Augsburg Confession, which is the primary confession of faith for Lutherans, as well as the Apology of the Augsburg Confession, written in response to Catholic objections to the Augsburg Confession. I must admit that in reading these two documents, I did not understand why the Catholic Church objected to them. I had to deduce what the Catholic position was through the arguments made in the Apology.
Fortunately, I eventually was able to find the “missing link” — the Catholic rebuttal to the Augsburg Confession which prompted the writing of the Apology of the Augsburg Confession. This document is known as The Roman Confutation against the Augsburg Confession and is available from the Lutheran Project Wittenberg. If you are curious why the Church said “No” to many of the Lutheran positions back in the 16th Century, I encourage you to read this document (available as a free PDF). It’s only 24 pages, and it proved to be a powerful influence on me becoming a Catholic. If you’d like to study it alongside the Lutheran documents, both the Augsburg Confession and the Apology are part of the Book of Concord, also available as a free PDF online.