Objection 1. It would seem that goodness of life is not required of those who receive Orders. For by Orders a man is ordained to the dispensation of the sacraments. But the sacraments can be administered by good and wicked. Therefore goodness of life is not requisite.
Objection 2. Further, the service of God in the sacraments is no greater than service offered to Him in the body. Now our Lord did not cast aside the sinful and notorious woman from rendering Him a bodily service (Luke 7). Therefore neither should the like be debarred from His service in the sacraments.
Objection 3. Further, by every grace a remedy is given against sin. Now those who are in sin should not be refused a remedy that may avail them. Since then grace is given in the sacrament of order, it would seem that this sacrament ought also to be conferred on sinners.
On the contrary, "Whosoever of the seed of Aaron throughout their families hath a blemish, he shall not offer bread to his God neither shall he approach to minister to him [Vulgate: 'Say to Aaron: Whosoever of thy seed,'etc.]" (Leviticus 21:17-18). Now "blemish signifies all kinds of vice" according to a gloss. Therefore he who is shackled by any vice should not be admitted to the ministry of Orders.
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.
I answer that, As Dionysius says (Eccl. Hier. iii), "even as the more subtle and clear essences, being filled by the outpouring of the solar radiance, like the sun enlighten other bodies with their brilliant light, so in all things pertaining to God a man must not dare to become a leader of others, unless in all his habits he be most deiform and godlike." Wherefore, since in every order a man is appointed to lead others in Divine things, he who being conscious of mortal sin presents himself for Orders is guilty of presumption and sins mortally. Consequently holiness of life is requisite for Orders, as a matter of precept, but not as essential to the sacrament; and if a wicked man be ordained, he receives the Order none the less, and yet with sin withal.
Reply to Objection 1. Just as the sinner dispenses sacraments validly, so does he receive validly the sacrament of Orders, and as he dispenses unworthily, even so he receives unworthily.
Reply to Objection 2. The service in point consisted only in the exercise of bodily homage, which even sinners can offer lawfully. It is different with the spiritual service to which the ordained are appointed, because thereby they are made to stand between God and the people. Wherefore they should shine with a good conscience before God, and with a good name before men.
Reply to Objection 3. 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.
Source: Summa Theologica (New Advent)