Q. Why are the biblical books of Luke and Acts separated if they were written by the same person?
A. St. Luke did indeed write both these books, apparently as two volumes of the same work (see Acts 1:1; the Greek word translated here as “book” is logos and can be rendered in English as “book” or “volume”). The Gospel ends at the point where Acts begins, with the ascension of Our Lord into heaven (see Luke 24:50-53; Acts 1:1-2). So in a way it would make sense to arrange the New Testament books so that Acts immediately follows Luke.
Nevertheless, a different criterion for arranging the books is employed here. From the beginning, the Gospels have been considered by the Church as the most important of the New Testament books, first in honor, “the heart of all the Scriptures, ‘because they are our principal source for the life and teaching of the Incarnate Word, our Savior’” (Catechism of the Catholic Church, 125).
Not surprisingly, the New Testament ordering reflects this primacy of the Gospels, placing them all together and before all the other books. So the Book of Acts could not be placed among the four Gospels in order to keep it next to Luke, with an arrangement such as Matthew, Mark, Luke, Acts, and John.
Why not simply place Luke as the last of the four Gospels so that Acts comes immediately after it? The ordering of the Gospels does in fact vary in the ancient lists that have survived, and some of those lists have placed Luke at the last, probably for that very reason. The order that seems to have been the most ancient, which St. Jerome used in the Vulgate (his fourth-century translation of the Bible into Latin), was Matthew, Mark, Luke, John. This order became the standard, most likely because it reflects the chronological order of the Gospels’ composition according to ancient tradition. That tradition suggests that Matthew was composed first and John last, with Mark and Luke in between. Given this order, Luke and Acts had to be separated.