This book (available online for free from that link) is the fuller version* of Luther's catechism (i.e. the basic text for introducing the tenets of a faith), and as such is probably one of the most influential key texts in the history of Protestant thought and practice. Lutheran** readers will likely be intimately familiar with it already but anyone with an interest in Christian history would find a lot to gain by reading it, and Christians of other denominations will discover in it a rich orthodox statement of how we are called, nay, privileged, to live by faith in the clear simple light of truth. I have no substantive theological or ethical bones to pick here - it is, from my perspective, a faithful and trustworthy testament and valuable for introductory teaching. It is consistently scripturally-grounded and remarkably well-written; Luther was not one to mince words and lays out these reflections on the Christian life and basic theological support for them in direct, accessible language.
Luther kicks the document off with a walk through the Ten Commandments, and here we come to our first quibble - he messes with them a bit for reasons that elude me. He has condensed the first and second commandments into one, so that we end up with it being a total statement of non-idolatrous monotheism, but with no discussion of the "who brought you out of the land of Egypt, out of the house of slavery" portion of these lines in the actual Bible (check it out); he has then kept the list counting to ten by separating the tenth commandment into two. Quite what he was doing in this I cannot say... I mean, so much for sola scriptura, right? But his actual reflections on the Ten are thoroughly helpful. A few minor quibbles - in the section on respecting one's mother and father he extends this commandment to political authority, which rankles me as an anarchist. Further on that point in the section on "thou shalt not kill" he seems to make the point that state authority is exempt from this particular moral absolute, which rankled my anarchism-senses even more. He does in this same section say that enabling death by privilege and neglect is just as condemnable as outright murder though, which is a take Peter Unger would heartily approve. In the section on adultery, we have this fascinating and disturbing quote when he is talking about the problematic outcome of Catholic insistence on clerical celibacy: "For no one has so little love and inclination to chastity as just those who because of great sanctity avoid marriage, and either indulge in open and shameless prostitution, or secretly do even worse, so that one dare not speak of it, as has, alas! been learned too fully." [italics mine.] We all know too well the Catholic Church's historical struggles with abusive paedophilia - part of me wonders whether this was already rearing its head five centuries ago to the point that it was unspoken but common knowledge. The section on theft is wonderfully expansive - a full Marxist analysis of this bit would yield some truly spectacular insights, I feel. The section on lying is morally robust but could be made a great deal stronger with the inclusion of some generally-considered epistemology and psychology. And the section[s] on envy take an unexpectedly objective, active view of this particular sin, which I was raised to think was more the mere subjective passive condition of indulging jealousy, but that Luther seems to say is when one makes actual decisive effort to acquire the property of another.
The sections walking through the Creed I have very little to say about - this is just hardcore solid uplifting theology communicated with a depth and a deftness I have seldom found elsewhere. His linguistic nuances when talking about "churchness" in the bit about the Holy Spirit are helpful, though I found his maintenance of institutional borders in those same paragraphs less so.
His opening reflections on the Lord's Prayer are just beautiful in style and empowering in substance. The discussion on the request for "our daily bread" furthermore is intriguingly and helpfully ecological and sociological in its scope.
The final section, dealing with the sacraments of baptism and the Lord's Supper, are also highly moving in their depictions of the spiritual realities contained within these ritual practices, and as a Quaker I found these parts rather convicting (Quakers in general do not formally practice either of these sacraments, as we take a more holistic symbolic view of their intent), in ways that bear further consideration on my part.
Overall this is an incredibly powerful statement of some of the basic tenets of a good Christian life, written by one of the most reputable sources thereupon. Definitely worth a read if that sounds like the kind of thing you'd benefit from. And who wouldn't? God's goodness shines through clearly on every page of this thing; it leaves one hungry for grace.
* Compare with the Small Catechism that I read yesterday. I gave minimal reflections on that as it made more sense to do that here when reading the principal longer document.
** I consider Lutheranism to be the burning bridge between the Catholic and Protestant communions - and this comes across in Luther's own doctrine, attitude and style.