9 See Deissmann, Bibelstudien, 13 sqq.
(1885), 1 -20; Deissmann, " Griechische Transkriptionen des Tetragrammaton," in Bibelstudien (1895), 1 -20; Blau, Das altji dische Zauberwesen, 1898.
Deissmann, Bible Studies, pp. 1-60, for this distinction between the genuine "letter" and the literary "epistle," as applied to the New Testament in particular.
The word is not found in pre-Christian writings except in the Septuagint, though as Deissmann has shown it is found on the Papyri as an official title for the village magistrates of Egypt and the members of the yEpovoia, or senate, of many towns in Asia Minor.
It is unlikely that he inserted this postscript from a feeling of ironical playfulness, to make the Galatians realize that, after the sternness of the early chapters, he was now treating them like children, "playfully hinting that surely the large letters will touch their hearts" (so Deissmann, Bible-Studies (1901), 346 f.).
Deissmann (Bible Studies) assigns to the category of letters all the Pauline writings as well as 2 and 3 John.