Who decides what passages from the Bible we read in church, and on what basis? The simple answer is that we buy the Lectionary every year, and this has the readings which we use. The long answer is a bit more complicated.
We know from Luke 4:17 that there was already a system of assigned readings (the book, if not the exact passage) in synagogues at the time of Jesus. Presumably Jesus' followers adopted the same pattern in their services, but we do not know, and we do not know how long it was before the writings of Christians - the letters and the gospels, for instance - came to be used.
All we know is that from the fourth century onwards, there were various attempts to standardize and systematize the readings in church services. In particular, the readings at Advent were chosen to reflect on Christ's coming, and in Lent and Holy Week to reflect on his passion and resurrection.
By the time of the Reformation, this was far from obvious. One of the Prefaces to the 1549 Book of Common Prayer, reissued in 1662, is worth quoting at length:
But these many years passed, this godly and decent order of the ancient Fathers hath been so altered, broken, and neglected, by planting in uncertain Stories, and Legends, with multitude of Responds, Verses, vain Repetitions, Commemorations, and Synodals; that commonly when any Book of the Bible was begun, after three or four Chapters were read out, all the rest were unread. And in this sort the Book of Isaiah was begun in Advent, and the Book of Genesis in Septuagesima; but they were only begun, and never read through: after like sort were other Books of holy Scripture used. ... And furthermore, notwithstanding that the ancient Fathers have divided the Psalms into seven portions, ... now of late time a few of them have been daily said, and the rest utterly omitted. Moreover, the number and hardness of the Rules called the Pie, and the manifold changings of the service, was the cause, that to turn the book only was so hard and intricate a matter, that many times there was more business to find out what should be read, than to read it when it was found out.
So the Book of Common Prayer came with a lectionary to ensure that the entire Bible was read, chapter by chapter, (well, almost - people were spared Leviticus 10, but might well find themselves sitting through Numbers 33!) in the course of a year, and the Book of Psalms in the course of a month. There were also special readings for use at Communion services on Sundays and Holy Days.
This arrangement continued for over 300 years, until 1871, when the first of a series of attempts was made to make the readings more self-contained and more relevant. (Remember that the Bible was only divided into chapters in the 13th century, and then not always logically!).
With special readings for Sundays, though, there was doubt over whether people were hearing enough of the Bible to "profit more and more in the knowledge of God", especially as many people no longer attended three services on that day. So a two-year lectionary was proposed. The Roman Catholic church had meanwhile woken up to the value of reading more of the Bible in church, and in 1969 produced a lectionary of their own, using a three-year cycle, based partly on the idea of the three years of our Lord's ministry portrayed in John.
In America, an ecumenical committee worked on unifying readings throughout the churches, and the result, in 1983, was the Common Lectionary. A revised version was issued in 1994, and the Church of England's Common Worship lectionary, which we follow, is based closely on that.
The Revised Common Lectionary runs in three year cycles - Advent Sunday this year marks the beginning of Year A again, with a particular emphasis on Matthew's Gospel. There are often two, sometimes more, options - we tend to use the "related" rather than the "continuous" set of readings. The object is that we hear the Bible being read in a balanced and relatively complete way!
Whatever the lectionary says, we can have different readings if there is a good reason. Perhaps we are having a series of sermons on prayer, or on miracles, and it makes sense to choose readings that illustrate the topic, rather than twist the topic out of the readings. But however the readings are chosen, their aim is to widen and deepen our knowledge of God's grace. So let us hear and read them, note them, and take them to heart.
HD