On the Third Office
In the third office we return to the head and remember the preaching of Christ to us. Moses prefigured this office when he ascended up the mountain to the Lord and received from him the tablets of the testament. And the Lord, whilst the entire people were listening, put forth his commandments, and Moses expounded the justices of the law, and the people responded that they would keep them all (Exod. XIX).
Thus the deacon goes towards the bishop, takes the book from the altar, and reads the Gospel, in which the divine precepts are, before the people. Then the bishop, preaching a sermon, expounds them to the people, and the people through the Kyrie eleyson, and the clerics through the Credo in unum Deum, give pledge that they will keep them all.
Now, in what way this was done by Christ and the apostles, we must briefly say.
De tertio officio.
In tertio officio ad caput reditur, et praedicatio Christi nobis ad memoriam reducitur. Hoc officium Moyses praefiguravit, quando in montem ad Dominum ascendit, et ab eo tabulas testamenti accepit. Dominusque, cuncto populo audiente, mandata proposuit, et Moyses iustitias legis exposuit, et populus se omnia servaturum respondit. Sic diaconus ad episcopum vadit, librum ab altari accipit, Evangelium, in quo divina praecepta sunt, coram populo legit. Episcopus sermonem faciens populo ea exponit, et populus per Kyrie eleyson, clerus autem per Credo in unum Deum, se spondet cuncta servaturum. Qualiter autem hoc per Christum et apostolos sit gestum, breviter est dicendum.