Objection 1. It would seem that he who raises the unworthy to orders commits no sin. For a bishop needs assistants appointed to the lesser offices. But he would be unable to find them in sufficient number, if he were to require of them such qualifications as the saints enumerate. Therefore if he raise some who are not qualified, he would seem to be excusable.
Objection 2. Further, the Church needs not only ministers for the dispensation of things spiritual, but also for the supervision of temporalities. But sometimes men without knowledge or holiness of life may be useful for the conduct of temporal affairs, either because of their worldly power, or on account of their natural industry. Therefore seemingly the like can be promoted without sin.
Objection 3. Further, everyone is bound to avoid sin, as far as he can. If therefore a bishop sins in promoting the unworthy, he is bound to take the utmost pains to know whether those who present themselves for Orders be worthy, by making a careful inquiry about their morals and knowledge, and yet seemingly this is not done anywhere.
On the contrary, It is worse to raise the wicked to the sacred ministry, than not to correct those who are raised already. But Heli sinned mortally by not correcting his sons for their wickedness; wherefore "he fell backwards... and died" (1 Samuel 4:18). 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"; and for the same reason all the spiritual things that he dispenses. Therefore he who promotes the unworthy sins mortally.
I answer that, Our Lord describes the faithful servant whom He has set "over His household to give them their measure of wheat." Hence he is guilty of unfaithfulness who gives any man Divine things above his measure: and whoso promotes the unworthy does this. 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.
Reply to Objection 1. 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.
Reply to Objection 2. Temporal things are not to be sought but for the sake of spiritual things. Wherefore all temporal advantage should count for nothing, and all gain be despised for the advancement of spiritual good.
Reply to Objection 3. It is at least required that the ordainer know that nothing contrary to holiness is in the candidate for ordination. But besides this he is required to take the greatest care, in proportion to the Order or office to be enjoined, so as to be certain of the qualifications of those to be promoted, at least from the testification of others. This is the meaning of the Apostle when he says (1 Timothy 5:22): "Impose not hands lightly on any man."
Source: Summa Theologica (New Advent)