The word “Duty” has been often used both by philosophers and in the holy Scriptures; from whence it is derived.
25. Since, therefore, the person concerned is one fit to write on the Duties, let us see whether the subject itself stands on the same ground, and whether this word is suitable only to the schools of the philosophers, or is also to be found in the sacred Scriptures. Beautifully has the Holy Spirit, as it happens, brought before us a passage in reading the Gospel to-day, as though He would urge us to write; whereby we are confirmed in our view, that the word officium, “duty,” may also be used with us. For when Zacharias the priest was struck dumb in the temple, and could not speak, it is said: “And it came to pass that as soon as the days of his duty [officii] were accomplished, he departed to his own house.” We read, therefore, that the word officium, “duty,” can be used by us.
26. And this is not inconsistent with reason, since we consider that the word officium (duty) is derived from efficere (to effect), and is formed with the change of one letter for the sake of euphony; or at any rate that you should do those things which injure [officiant] no one, but benefit all.
Reproduced in full. Translation: Nicene and Post-Nicene Fathers, series 2, vol. 10 (Schaff and Wace, 1896), public domain (Public domain).