Before I get into that, a little preface on the Preface might be in order. We, Episcopalians, take the Book of Common Prayer for granted. As is common in most Church life, the things we do, we often do without really stopping to think why we do things a certain way. "We've always done it this way!" In some ways, this is actually a true statement, at least as it pertains to how worship came to be governed in what became the Church of England via the English Reformation and the Church of England's theological descendants like the Episcopal Church in the USA.
The Book of Common Prayer was the brainchild of a man named Thomas Cranmer. I have written on this before at some length. (I was halfway through seminary before I realized the "n" comes before the "m" in the spelling of his name; to this day Cramner seems to roll of my tongue easier than Cranmer.) Cranmer was the first Protestant Archbishop of Canterbury, appointed by Henry VIII after Henry VIII's break with Rome.
Cranmer as a theologian is something of an enigma. He was just as much a politician as he was a liturgist. As such, it is virtually impossible to discern where his own person theological beliefs begin and where the theology that he thought the King wanted to hear ends. Cranmer was a master at knowing which way the wind was blowing and always played his personal theological cards very close to his vest, so to speak.
Most of what we know about Cranmer's personal theology we have to surmise either from his monumental works of the first two Books of Common Prayer and his personal correspondences. All of which, depending on whom was in power, contradict each other on many finer points of theological doctrine. We can basically assert that Cranmer was a reformer. He was adamant about some of the Protestant ideals like having the Mass in a language people can understand and making worship accessible to everyday Christians who had everyday jobs and were not professional clergy who got paid to pray the offices all day.
Other than that, it is largely anyone's guess on what he personally thought about issues like Justification by Faith Alone and Calvinist notions of predestination. As to exactly how much of Catholic theology held sway with him, no one really knows. He did not keep a personal spiritual journal nor did he write his own Systematic Theology. Generally, historians tend to think he was more progressively Protestant than he let on, at least to Henry VIII, but the very fact that he had no qualms about using Catholic missals and worship resources as primary sources for his Book of Common Prayer (which came out after Henry VIII had died and his more Protestant son was on the throne), he obviously saw some value in Catholic worship, if not Catholic theology. He did ultimately stand up for Protestantism at the end of his wife when we was burned at the stake after a brief recanting to Catholic authorities under Mary Tudor. As I said, it is really anyone's guess as to what he personally believed on specific points of Reformation doctrine.
In the early years of Edward VI, Cranmer was given free reign to come up with liturgical reform. Henry VIII was extremely liturgically conservative. If you went to church on the Sunday after Henry VIII died, you would have seen a Medieval Catholic Mass in Latin. An English bible would have been chained to a desk somewhere in the building, but that would about the only clue you were in an English "Protestant" church. Of course, if you attended a minster (a church attached to a monastery run by the monastery clergy), you probably wouldn't have seen a church service at all as Henry VIII closed all the monasteries and turned their lands and building into Crown holdings. But in an everyday parish church, you would have seen a medieval Latin mass.
So, with Henry VIII out of the picture, and a much more Protestant Edward VI on the throne, Cranmer would have had freer reign to purvey much more Protestant worship. The first edition of his Prayerbook, however, from 1549 is extremely Catholic in worship and theology. It still uses the term "sacrifice," and its form is still very close to the Catholic Sarum Rite missal that was used in and around Salisbury. Perhaps Cranmer's personal views were in a state of flux and evolving, or perhaps he was simply trying to "read the waters" and come up with what he thought the new monarch wanted to hear. Edward VI actually despised the first edition of the Prayerbook as "too Catholick" and, as such, the second Book of Common Prayer from 1552 is much more Protestant in form and theology, much more to Edward VI's liking. But that is a topic for another time.
With this background, let us take a look at the first paragraph of the original preface of the 1549 Book of Common Prayer and see how Cranmer himself justifies his Book of Common Prayer. For our purposes, the following is the modernized spelling version from the Historical Documents Section of the 1979 American Book of Common Prayer:
"There was never any thing by the wit of man so well devised, or so sure established, which
in continuance of time hath not been corrupted: as, among other things, it may plainly
appear by the common prayers in the Church, commonly called Divine Service: the first
original and ground whereof, if a man would search out by the ancient fathers, he shall find,
that the same was not ordained, but of a good purpose, and for a great advancement of
godliness: For they so ordered the matter, that all the whole Bible (or the greatest part
thereof) should be read over once in the year, intending thereby, that the Clergy, and
especially such as were Ministers of the congregation, should (by often reading, and
meditation of God's word) be stirred up to godliness themselves, and be more able to exhort
others by wholesome doctrine, and to confute them that were adversaries to the truth. And
further, that the people (by daily hearing of holy Scripture read in the Church) should
continually profit more and more in the knowledge of God, and be the more inflamed with
the love of his true religion."
-1st Paragraph of the Preface to the first Book of Common Prayer, AD 1549
Modern readers will immediately note that this paragraph is, in fact, only two complete sentences. The first sentence is 10 lines long, but completely grammatically correct. It is not a run-on sentence. Modern Americans simply do not write (or can not even largely comprehend) sentences of this length and magnitude. We like short, terse Twitter feeds. I think it is a good exercise, however, to actually take long sentences like this and ponder them. If you have a 10 feet length of butcher paper, try to diagram this sentence. It's well worth the time.
For clarity, I will break this down. Today let us focus on the first two lines, which is a logical and digestible clause: "There was never any thing by the wit of man so well devised, or so sure established, which
in continuance of time hath not been corrupted:" That is to say, everything gets corrupted over time. That phrase, in a nutshell, explains the Protestant Reformation from the Protestant point of view: People forget why they are doing things; people forget what they are doing (and how to do it); and then people just forget the whole point of it all to begin with. Reform is therefore, so the Protestant logic goes, needed to get people "back on task," as my first grade teacher used to say to students who were daydreaming. Most Protestants believed that if the Church could simply get back to the way the Disciples did things, to re-attain that golden age of Early, Apostolic Christianity by peeling away the corruptions and distortions that man (i.e. the Catholic Church) had added to distort the Message of the Gospel over the centuries, then all of Christianity's problems would be solved.
I, personally, disagree with that logic, at least in part, for a number of reasons. Firstly, I don't believe in the myth that the Early Church was somehow the Gold Standard for all subsequent generations of Christians. If you read the goings-on that Paul talks about in his Epistles or the fights described in the Book of Acts, the Early Church was, frankly, messed up. While I think we can learn much from them, I think trying to emulate the Early Church in every way, shape, and form is impossible because we would have to re-create the old Roman Empire and its culture. That's simply not possible either.
Finally, there is a difference between man made tradition and Holy Tradition. God continued to act and speak to the Church in the centuries after the Apostolic era. Protestants act like God somehow walked off the scene for 1000+ years and only came back when the Reformers came on the scene. God was still speaking, and Christians were still trying to follow God as best they could. Occasional reform does need to be made to peel off the non-essential layers that man has added, but we need to be very careful that we don't throw out the baby with the bath water. While the Revelation of the Bible is unique, we also have to be open to the fact that the Holy Spirit was at work in prior generations, and maybe some of that Holy Tradition that the Church learned during prior periods needs to be retained. If you do not learn from history, you are bound to repeat it, so the proverb goes.
As such, I think what Cranmer's motives were for the first Book of Common Prayer was an attempt to peel off the man made tradition as it pertains to the true worship of God but retain that Spirit filled wisdom and worship that God revealed to Christians in ages past. One can argue how successful or not he was in accomplishing this, but the preface to the Book of the Common Prayer does state this as a primary goal in justifying this liturgical reform.