The reason this is a wonderful practice is that this type of preaching demonstrates the unity of Scripture. In reality, different texts from various authors and generations testify to a unified body of truth. This manner of preaching ensures that we develop a Christian mind and understanding of what this body of truth is. Another way of thinking about this is that we consider the trees in our first service, since we focus on a particular text of Scripture. And in our second (the Sunday School Service), we consider the forest, since we learn how a particular doctrine arises from many biblical texts.
—Zac Wyse, “Why Do You Preach From the Catechism?”