Thomas Aquinas

“ 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. ”

“ Wherefore some are raised to the priesthood, to whom the first act alone is deputed, for instance those religious who are not empowered with the care of souls. The law is not sought at the mouth of these, they are required only for the celebration of the sacraments; and consequently it is enough for them to have such knowledge as enables them to observe rightly those things that regard the celebration of the sacrament. Others are raised to exercise the other act which is over the mystical body of Christ, and it is at the mouth of these that the people seek the law ”

“ Further, holiness of life is required in one who receives an order, that he may be qualified to exercise it. Now a man sins mortally if he present himself for orders in mortal sin. Much more therefore does he sin mortally whenever he exercises his order. ”

“ Certain medicines require a robust constitution, else it is mortally dangerous to take them; others can be given to the weakly. So too in spiritual things certain sacraments are ordained as remedies for sin, and the like are to be given to sinners, as Baptism and Penance, while others, which confer the perfection of grace, require a man made strong by grace.
Art. 2 - Whether knowledge of all Holy Writ is required? Objection 1: It would seem that knowledge of all Holy Writ is required. For one from whose lips we seek the law, should have knowledge of the law. ”

“ Wherefore whoever fulfills unworthily the duties of his order follows unjustly after that which is just, and acts contrary to a precept of the law, and thereby sins mortally. Now anyone who exercises a sacred office in mortal sin, without doubt does so unworthily. ”

“ 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. ”

“ God never so abandons His Church that apt ministers are not to be found sufficient for the needs of the people, if the worthy be promoted and the unworthy set aside. And though it were impossible to find as many ministers as there are now, it were better to have few good ministers than many bad ones, as the blessed Clement declares in his second epistle to James the brother of the Lord. ”

“ Further, Jerome commenting on the words of Titus 2:15, "Let no man despise thee," says that "not only should bishops, priests, and deacons take very great care to be examples of speech and conduct to those over whom they are placed, but also the lower grades, and without exception all who serve the household of God, since it is most disastrous to the Church if the laity be better than the clergy." Therefore holiness of life is requisite in all the Orders. ”

“ It seems presumptuous for such a man, one to wit who is not enlightened, to lay hands on priestly things; he is not afraid nor ashamed, all unworthy that he is to take part in Divine things, with the thought that God does not see what he sees in himself ”

“ Therefore he who promotes the unworthy does not escape sin.
Further, spiritual things must be set before temporal things in the Church. Now a man would commit a mortal sin were he knowingly to endanger the temporalities of the Church. Much more therefore is it a mortal sin to endanger spiritual things. But whoever promotes the unworthy endangers spiritual things, since according to Gregory (Hom. xii in Evang.) "if a man's life is contemptible, his preaching is liable to be despised" ”

“ Wherefore he commits a mortal crime, as being unfaithful to his sovereign Lord, especially since this is detrimental to the Church and to the Divine honor which is promoted by good ministers. For a man would be unfaithful to his earthly lord were he to place unworthy subjects in his offices. ”