When scripture passages are read in the early part of the worship service, we call them "The Lessons". The first selection will be from the Old Testament, the second from the Psalms (said as poetry-prayer by the people), the third from the Epistles (the new Testament letters), and the last, a passage from the Gospels (Matthew, Mark, Luke or John).
These are actually not lessons as we use the word today but are scripture selections from which it is hoped we will learn a lesson. If we pay close attention, we will notice that the selections have a common theme. Most often the sermon will expand on the theme.
This custom of reading scripture passages comes to us from the early Christians who used the services of the Jewish people as their pattern. Gradually, as they added readings from the Christian writings to those from Hebrew scripture, they began to make what they called "lectionaries"; that is, schedules which provided systematic patters for the readings.
If you turn to the back of your Book of Common Prayer, you will see that the Sunday scripture selections are arranged in three-year cycles. (There is a two-year cycle for the daily office.) An enormous amount of biblical text is read and heard over this period of time. When others boast that they are "Bible churches", Episcopalians can note the shear amount of scripture that is used within a single service.
The question often arises as to which is better: a spontaneous selection or using fixed pre-selected readings. Each group will point to their way as being best, but the truth is that both ways have advantages. In the free selection of scripture, a service can reflect the immediate interests of the minister and the people. The Episcopal sermon has some flexibility in being able to comment on any theme found within the selected passages. A lectionary has the advantage of discipline and direction. Over a period of time, we will not become stuck on one or two topics but will hear all of God's word in its full richness and diversity.