There are literally dozens of groups out there, who claim the mantle of “Lutheran,” and many of them define the term differently. Historically, particularly in Germany and Scandinavia where the Lutheran Reformation took hold, and from whence the Lutheran churches of America find most of their roots, what made a church Lutheran was subscription to the Lutheran Confessions. The Lutheran Confessions made clear doctrinal statements regarding the teaching of Holy Scripture, reflected the teaching of Luther during his time, and the generation immediately after his death.
The Lutheran Confessions are traditionally bound together in the 1580 Book of Concord. While not every Lutheran church places the same emphasis on all of the documents within that corpus, these have traditionally defined the way Lutheran Christians walk together under Holy Scripture. The most central of those documents, are the Ecumenical Creeds (Apostles, Nicene, and Athanasian,) which are the ancient symbols of Christian unity from the 4th and 5th centuries AD.
The Augsburg Confession and the Apology state Lutheran positions on the controversies of the early 16 century, placing the Holy Scriptures as the sole canon (or rule) of Christian faith, and declaring the centrality of the Doctrine of Justification by Grace through Faith in Christ Alone. The Treatise on the Power and Primacy of the Pope, The Smalcald Articles, and the Formula of Concord present clarifications, explanations, and extensions of the doctrines declared at Augsburg.
The twoCatechisms of Martin Luther (Small and Large) form the daily piety and instruction of the Christian faithful.
As Lutheran Christians, we receive the teachings of the Lutheran Confessions as normative in our churches, because they are clear and true expositions of Holy Scripture, and the historic Christian faith.