God wished to produce His works in likeness to Himself, as far as possible, in order that they might be perfect, and that He might be known through them. Hence, that He might be portrayed in His works, not only according to what He is in Himself, but also according as He acts on others, He laid this natural law on all things, that last things should be reduced and perfected by middle things, and middle things by the first, as Dionysius says (Eccl. Hier. v). Wherefore that this beauty might not be lacking to the Church, He established Order in her so that some should deliver the sacraments to others, being thus made like to God in their own way, as co-operating with God; even as in the natural body, some members act on others. - St. Thomas Aquinas

The Sacrament of Holy Orders is the sacrament instituted by Christ to re-present Himself in various ways. The three degrees of Holy Orders are the Diaconate, the Priesthood, and the Episcopacy.