Objection 1. It would seem that the Orders ought not to be divided into those that are sacred and those that are not. For all the Orders are sacraments, and all the sacraments are sacred. Therefore all the Orders are sacred.
Objection 2. Further, by the Orders of the Church a man is not appointed to any other than Divine offices. Now all these are sacred. Therefore all the Orders also are sacred.
On the contrary, The sacred Orders are an impediment to the contracting of marriage and annul the marriage that is already contracted. But the four lower orders neither impede the contracting nor annul the contract. Therefore these are not sacred Orders.
I answer that, An Order is said to be sacred in two ways. First, in itself, and thus every order is sacred, since it is a sacrament. Secondly, by reason of the matter about which it exercises an act, and thus an Order is called sacred, if it exercises an act about some consecrated thing. In this sense there are only three sacred Orders, namely the priesthood and diaconate, which exercise an act about the consecrated body and blood of Christ, and the subdiaconate, which exercises an act about the consecrated vessels. Wherefore continency is enjoined them, that they who handle holy things may themselves be holy and clean.
This suffices for the Replies to the Objections.
Source: Summa Theologica (New Advent)