The Epistle follows immediately after the Collect, and is the first of the two Biblical readings Proper to the particular feast day (the second being the Gospel). The term comes from the Greek epistole and Latin epistola, meaning “letter”—many were indeed drawn from the letters of Paul to the Apostles. But the Old Testament was also mined for Epistle readings, as is the case for the feast of St. Donatian, whose Epistle reading is drawn from the Book of Ecclesiasticus and, like the Introit, offers praise to a great priest.
It was the subdeacon’s responsibility to deliver the Epistle from the south, or right side (“Epistle side”) of the altar; he relied on a simple recitation tone with just two cadential formulae, one for full stops and one for half closes. The presentation of the Epistle is thus musically simpler than the Gospel, and is also accompanied by less ceremony than surrounded the Gospel reading.
For more information, see:
Huglo, Michel, and James W. McKinnon. 2001 “Epistle.” Grove Music Online. 16 Jul. 2018. www.grovemusiconline