This point is admitted even by Protestant scholars: Edmon L. Gallagher notes that, for Christians, the LXX tradition was never restricted merely to the Greek Pentateuch (emphasis mine):
The translation for Christians encompasses the entire Old Testament, or, at least, more than the Greek Pentateuch. Justin [Martyr], for instance, refers to the Septuagint translation of Isaiah (Dial. 70–73). This view of the Septuagint becomes immediately universal for Christians. Irenaeus, later in the second century, relates the translation story, affirming the inspiration of the translators (in line with Philo) but also describing the translation of Isaiah as a part of the Septuagint (Haer. 3.21.1–3)
— Translation of The Seventy (Section I.3)
This is precisely why the term LXX is being used in this series in its broader Patristic sense. As I laid out in the opening chapter, the Old Testament (OT) received and used by the early Church was not identical in scope to the later Rabbinic canon, nor was it governed by the Masoretic textual tradition. It was the Greek OT, the LXX, which passed into the hands of the Church and became the Scriptures that they used. What is decidedly more controversial is whether Western Christians are willing to let that fact govern their understanding of the OT and the Canon as a whole.
