Is there a trend in the meaning of religious text titles?

“Bible” comes from the Greek word for “the books,” but I think you’ve read the rest of the pattern a bit selectively.

The Tanakh or Hebrew Bible (what Christians call the Old Testament) has traditionally been seen as having three parts: the Torah (the first five books, traditionally regarded as having been written by Moses), the Prophets (which are also traditionally divided into the Former Prophets, comprising the books of Joshua, Judges, Samuel, and Kings, and the Latter Prophets), and the Writings, as a catchall for the rest.

While the New Testament does have four gospels, it also has the Acts of the Apostles, a collection of Pauline epistles, the so-called Catholic Epistles of Peter, John, and James, and an apocalypse (the Revelation of John). When one looks at the history of the New Testament, it’s important to remember that its component parts were never written with the intent of being made a part of a canon consisting of all these other writings. Canonization happened throughout the course of the first and second centuries, with the status of some books (notably, Revelation) being highly disputed, while others like some of the Catholic Epistles didn’t have much in the way of historical provenance or an origin story. It’s rather a collection of writings agreed on after the fact, so that none of the authors had in mind any categorizing scheme that would shine through the finished product.

“Testament” comes through the Latin Testamentum, which might mean “word” in a poetical sense but more literally means something like “testimony.”