Happy Feast of Saint Pius V. The following is from the Catechism of the Council of Trent.
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Importance of Holy Orders
If one attentively considers the nature and essence of the other Sacraments, it will readily be seen that they all depend on the Sacrament of Orders to such an extent that without it some of them could not be constituted or administered at all; while others would be deprived of all their solemn ceremonies, as well as of a certain part of the religious respect and exterior honor accorded to them.
The Sacrament’s Dignity
Bishops and priests being, as they are, God's interpreters and ambassadors, empowered in His name to teach mankind the divine law and the rules of conduct, and holding, as they do, His place on earth, it is evident that no nobler function than theirs can be imagined.
In all ages, priests have been held in the highest honor; yet the priests of the New Testament far exceed all others. For the power of consecrating and offering the Body and Blood of our Lord and of forgiving sins, which has been conferred on them, not only has nothing equal or like to it on earth, but even surpasses human reason and understanding.
Not For Everyone
The burden of this great office, therefore, should not be rashly imposed on anyone, but is to be conferred on those only who by their holiness of life, their knowledge, faith and prudence, are able to bear it. Let no one take the honor to himself, but he that is called by God as Aaron was; and they are called by God who are called by the lawful ministers of His Church.
The Power and Its Greatness
The power of orders not only embraces the power of consecrating the Eucharist, but also fits and prepares the souls of men for its reception. It also embraces all else that can have any reference to the Eucharist. Christ our Lord ... was a Priest, not according to the order of Aaron, but according to the order of Melchisedech. For He it is Who, Himself endowed with the supreme power of granting grace and remitting sins, left to His Church this power, although He limited it in extent and attached it to the Sacraments.
The chief and most necessary quality requisite in him who is to be ordained a priest is that he be recommended by integrity of life and morals. The priest is bound to give to others the example of a holy and innocent life. There is required of the priest not only that knowledge which concerns the use and administration of the Sacraments; but he should also be versed in the science of Sacred Scripture, so as to be able to instruct the people in the mysteries of the Christian faith and the precepts of the divine law, lead them to piety and virtue, and reclaim them from sin. The priest's duties are twofold. The first is to consecrate and administer the Sacraments properly; the second is to instruct the people entrusted to him in all that they must know or do in order to be saved.