Objection 1. It would seem that we ought not to distinguish several Orders. For the greater a power is, the less is it multiplied. Now this sacrament ranks above the others in so far as it places its recipients in a degree above other persons. Since then the other sacraments are not divided into several of which the whole is predicated, neither ought this sacrament to be divided into several Orders.
Objection 2. Further, if it be divided, the parts of the division are either integral or subjective. But they are not integral, for then the whole would not be predicated of them. Therefore it is a division into subjective parts. Now subjective parts can have the remote genus predicated of them in the plural in the same way as the proximate genus; thus man and ass are several animals, and are several animated bodies. Therefore also priesthood and diaconate, as they are several Orders, even so are several sacraments, since sacrament is the genus, so to speak, in respect of Orders.
Objection 3. Further, according to the Philosopher (Ethic. viii, 10) the form of authority in which one alone governs is a better government of the common weal than aristocracy, where different persons occupy different offices. But the government of the Church should be the best of all. Therefore in the Church there should be no distinction of Orders for different acts, but the whole power should reside in one person; and consequently there ought to be only one Order.
On the contrary, The Church is Christ's mystical body, like to our natural body, according to the Apostle (Romans 12:5; 1 Corinthians 12:12-27; Ephesians 1:22-23; Colossians 1:24). Now in the natural body there are various offices of the members. Therefore in the Church also there should be various Orders.
Further, the ministry of the New Testament is superior to that of the Old Testament (2 Corinthians 3). Now in the Old Testament not only the priests, but also their ministers, the Levites, were consecrated. Therefore likewise in the New Testament not only the priests but also their ministers should be consecrated by the sacrament of Order; and consequently there ought to be several Orders.
I answer that, Multiplicity of Orders was introduced into the Church for three reasons. First to show forth the wisdom of God, which is reflected in the orderly distinction of things both natural and spiritual. This is signified in the statement of 1 Kings 10:4, that "when the queen of Saba saw... the order of" Solomon's "servants... she had no longer any spirit in her," for she was breathless from admiration of his wisdom. Secondly, in order to succor human weakness, because it would be impossible for one man, without his being heavily burdened, to fulfill all things pertaining to the Divine mysteries; and so various orders are severally appointed to the various offices; and this is shown by the Lord giving Moses seventy ancients to assist him. Thirdly, that men may be given a broader way for advancing (to perfection), seeing that the various duties are divided among many men, so that all become the co-operators of God; than which nothing is more God-like, as Dionysius says (Eccl. Hier. iii).
Reply to Objection 1. The other sacraments are given that certain effects may be received; but this sacrament is given chiefly that certain acts may be performed. Hence it behooves the sacrament of Order to be differentiated according to the diversity of acts, even as powers are differentiated by their acts.
Reply to Objection 2. The division of Order is not that of an integral whole into its parts, nor of a universal whole, but of a potential whole, the nature of which is that the notion of the whole is found to be complete in one part, but in the others by some participation thereof. Thus it is here: for the entire fulness of the sacrament is in one Order, namely the priesthood, while in the other sacraments there is a participation of Order. And this is signified by the Lord saying (Numbers 11:17): "I will take of thy spirit and give to them, that they may bear with thee the burden of the people." Therefore all the Orders are one sacrament.
Reply to Objection 3. In a kingdom, although the entire fulness of power resides in the king, this does not exclude the ministers having a power which is a participation of the kingly power. It is the same in Order. In the aristocratic form of government, on the contrary, the fulness of power resides in no one, but in all.
Source: Summa Theologica (New Advent)