Catholics and many Protestants follow a similar lectionary. The central reading on Sunday is the Gospel, where we hear about the life and teachings of Jesus. We stand during this reading to acknowledge its significance. Each year, we read through most of Matthew, Mark, or Luke. Readings from John are interspersed through Christmas, Easter, and other holiday seasons.
Psalms are laments and praises cried and sung by the faithful throughout history.
The psalm is chosen as a response to the Old Testament reading and is spoken by the whole congregation as a prayer. The epistle does not necessarily reflect the theme of the Gospel. Instead, passages from a letter such as Corinthians are read sequentially, starting at the beginning and continuing each week through the entire letter. In many respects, this question gets to the heart of what the Bible is. The Bible is a collection of readings that have been authorized for use in corporate worship.