Everywhere rank exists.
1 Let us now take this into consideration: The power ordered to some principal effect by nature has under it inferior powers which serve it. This is especially clear in the arts, for the are which introduces into a thing its artificial form is served by the are which prepares the material; in turn, the one which introduces the form serves the are to which the end of the artificial thing belongs; the one in turn ordered to a further end serves the one ordered to the ultimate end. just so, the are of the, wood-cutter serves that of the ship-builder, and the latter that of navigation, and this in turn the are of economy, or of warfare, or something of this sort, since the navigator’s are can be ordered to different ends.
Since, then, the power of orders is principally ordered to consecrating the body of Christ and dispensing it to the faithful, and to cleansing the faithful from their sins, there must be some principal order whose power extends principally to this; this is the order of the priesthood; and there must be other orders which serve this one by preparing the material, and these are the ministerial orders. Now, since the priestly power, as was said, is extended to two things—namely, the consecration of the body of Christ and making the faithful ready for the Eucharist by absolution from their sins—the lesser orders must serve the priestly power either in both of these things, or else in one or the other. And, manifestly, an order is superior among the inferior orders by just as much as it serves the superior order in many things or in some worthier one.
2 Therefore, the lowest orders serve the priestly order merely in the preparation of the people: doorkeepers, by actually keeping unbelievers out of the gathering of believers; readers, by instructing catechumens in the rudiments of the faith—hence, the Old Testament Scripture is assigned them for reading, exorcists, however, by cleansing those who are already instructed, but to some extent are obstructed by the devil from the reception of the sacraments.
3 The superior orders serve the priestly order both in the preparation of the people and in the consummation of the sacrament. Acolytes have supervision over vessels which are not sacred and in which the material of the sacrament is prepared; hence, the cruets are handed to them during their ordination. Subdeacons, however, have supervision over sacred vessels and the disposal of material not yet consecrated.
But deacons, beyond this, have supervision over consecrated material in that they dispense the blood of Christ to the faithful. Accordingly, these three orders—the priesthood, the diaconate, and the subdiaconate—are called sacred orders because they receive a ministry over something sacred. The superior orders serve also in the preparation of the people. Hence, deacons are entrusted with the Gospel teaching to present it to the people, and subdeacons with the apostolic teaching; acolytes are entrusted with the performance in each of the two cases with what belongs to the solemnity of the teaching, namely, that they carry the lights and administer tasks of this kind.