Objection 1. It would seem that a man obtains the degrees of order by the mere merit of his life. For, according to Chrysostom [Hom. xliii in the Opus Imperfectum, wrongly ascribed to St. John Chrysostom], "not every priest is a saint, but every saint is a priest." Now a man becomes a saint by the merit of his life. Consequently he thereby also becomes a priest, and "a fortiori" has he the other Orders.
Objection 2. Further, in natural things, men obtain a higher degree from the very fact that they are near God, and have a greater share of His favors, as Dionysius says (Eccl. Hier. iv). Now it is by merit of holiness and knowledge that a man approaches nearer to God and receives more of His favors. Therefore by this alone he is raised to the degree of Orders.
On the contrary, Holiness once possessed can be lost. But when once a man is ordained he never loses his order. Therefore order does not consist in the mere merit of holiness.
I answer that, A cause should be proportionate to its effect. And consequently as in Christ, from Whom grace comes down on all men, there must needs be fulness of grace; so in the ministers of the Church, to whom it belongs, not to give grace, but to give the sacraments of grace, the degree of order does not result from their having grace, but from their participating in a sacrament of grace.
Reply to Objection 1. Chrysostom is speaking of the priest in reference to the reason for which he is so called, the word "sacerdos" signifying dispenser of holy things [sacra dans]: for in this sense every righteous man, in so far as he assists others by the sacraments, may be called a priest. But he is not speaking according to the actual meaning of the words; for this word "sacerdos" [priest] is employed to signify one who gives sacred things by dispensing the sacraments.
Reply to Objection 2. Natural things acquire a degree of superiority over others, from the fact that they are able to act on them by virtue of their form; wherefore from the very fact that they have a higher form, they obtain a higher degree. But the ministers of the Church are placed over others, not to confer anything on them by virtue of their own holiness (for this belongs to God alone), but as ministers, and as instruments, so to say, of the outpouring from the Head to the members. Hence the comparison fails as regards the dignity of Order, although it applies as to congruity.
Source: Summa Theologica (New Advent)