The following is taken from https://www.bookofconcord.org. We highly recommend this site, particularly the Frequently Asked Questions section.
A Brief Introduction to the Book of Concord
The Book of Concord contains documents which Christians from the fourth to the 16th century A.D. explained what they believed and taught on the basis of the Holy Scriptures. It includes, first, the three creeds which originated in the ancient church, the Apostles’ Creed, the Nicene Creed, and the Athanasian Creed. It contains, secondly, the Reformation writings known as the Augsburg Confession, the Apology of the Augsburg Confession, the Smalcald Articles, the Treatise on the Power and Primacy of the Pope, Luther’s Small and Large Catechisms, and the Formula of Concord. The Catechisms and the Smalcald Articles came from the pen of Martin Luther; the Augsburg Confession, its Apology, and the Treatise were written by Luther’s co-worker, the scholarly Phillip Melanchthon; the Formula of Concord was given its final form chiefly by Jacob Andreae, Martin Chemnitz, and Nickolaus Selnecker.
I. The Creeds:
The historical background of the documents in The Book of Concord is very interesting. The Apostles’ Creed was not composed by the apostles but is a faithful confession of apostolic doctrine; it is a “daughter” of the creed used by early Christians in Rome. The wording of the creed, as we confess it today, can be traced to southern Gaul (France).
The Nicene Creed is spoken today in many Lutheran congregations on Communion Sundays or festive occasions but its history presents some problems. According to popular belief it was formulated by the Council of Nicea A.D. 325 and revised by the Council of Constantinople A.D. 381. But this theory has been challenged. Another theory is that the creed had its roots in the creed of Jerusalem adopted by Epiphanius of Cyprus, then came to the Council of Constantinople via Syedra in Pamphilia, was used in connection with the consecration of a new bishop, found its way into the council minutes, and was mistakenly believed by the Council of Chalcedon in A.D. 451 to have been prepared 70 years earlier at the council in whose minutes it appeared. (One addition to the original formula, namely that the Holy Spirit proceeds from the Father “and the Son” is Western in origin and appears as early as the Synod of Toledo in A.D. 589.)
The Athanasian Creed is the longest of the three. Though included in The Lutheran Hymnal, many congregations use it only on Trinity Sunday (or the First Sunday after Pentecost). It is named after Athanasius, the great fourth-century champion of Orthodoxy against heretics who denied the deity of Christ. The creed originated in southern Gaul, probably about the middle of the sixth century.
The inclusion of the three ancient creeds in The Book of Concord indicates that Lutherans are not a sect but that they embrace and confess the ancient and orthodox faith.
II. The Lutheran Confessions:
Among the particular Lutheran Confessions the two catechisms of Dr. Martin Luther are the earliest. Luther published them in the spring of 1529 to help Pastors and parents give instruction in the chief parts of Christian doctrine.
The Augsburg Confession was written by Melanchthon in 1530. Emperor Charles V had invited the Lutheran princes and theologians to attend a meeting of government leaders at Augsburg. He wanted to discuss how the religious controversy in his empire could be settled, so that German Lutheran princes would join the imperial forces to keep the Turks out of Europe. The Augsburg Confession is composed of several documents which already existed but which were combined by Melanchthon to give a clear but conciliatory summary of the teachings and practices of the Lutheran pastors and congregations. It is to this day the basic Lutheran confession.
The Apology of the Augsburg Confession was published in 1531. After the Augsburg Confession had been read to the emperor, a committee of Roman catholic theologians prepared a reply called the confutation. The Apology defends the Augsburg Confession against the accusations of the Confutation.
The Smalcald Articles were written by Luther in late 1536. On June 4, 1536, Pope Paul III announced that a council would be held in Mantua beginning May 8, 1537, to deal with the concerns of the Protestants. The elector (or prince) of Saxony requested Luther to prepare some articles for discussion at the council. Luther indicated on which points Lutherans would stand fast and on which points a compromise might be possible. These articles were never used for their intended purpose, but Lutherans at once recognized their value as a statement of pure evangelical doctrine, and they were therefore included in The Book of Concord.
The Treatise on the Power and Primacy of the Pope was prepared by Melanchthon at the Protestant meeting at Smalcald in 1537 where Luther’s articles were to be discussed but, partly because Luther became ill, were never publicly presented to the assembly. Instead Melanchthon was requested to prepare a treatise which actually is an appendix to the Augsburg Confession.
The Formula of Concord was written a generation after Luther’s death. Serious controversies had arisen among theologians of the Augsburg Confession which threatened the very life of the Reformation. The Formula of Concord deals with these dissensions and presents the sound Biblical doctrine on the disputed issues.
No doubt much will – and should – be made of The Book of Concord as we observe its 400th anniversary. But the most worthy and God-pleasing way for Lutherans of the 20th century to commemorate the publication of The Book of Concord would be to engage in earnest study of the precious Confessions it contains and to commit themselves anew to the glorious truths of God’s Word which they teach.
A professor a generation ago described the significance of the confessional writings perhaps best of all. Prof. William Arndt wrote in the Concordia Theological Monthly:
The Confessions are the brightest jewel in the crown of the Lutheran Church. In speaking of our Confessions we dwell on facts that should make the heart of every Lutheran swell with joy and thanksgiving. We look here on one of the brightest pages of our history as a church. It is true, I admit, that the laurels of our fathers must not become the soft bed of the children on which they repose in sweet indolence, and it may be that there is somebody who speaks about the achievements of his ancestors to such an extent that he entirely forgets about the plowing, harvesting, and threshing which he himself ought to do. But my plea is that we do not become so occupied with our daily tasks in the churches that we forget the magnificent treasures which are furnished us in our Confessions. To study them, to read them frequently, to ponder their content, is like traveling, in a mountain country where the air is pure, the brooks sparkle, the birds sing their most beautiful songs, and the clatter of the noisy streets cannot disturb and intrude.