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Section from Pope Paul VI's Apostolic Letter 'Summi Dei Verbum'
November 4th 1963
On the occasion of the fourth centenary of the Establishment of Seminaries
by the Council of Trent
Nature of vocation. Its first source: God. Necessity of prayer
The first duty then that devolves on all Christians in regard to priestly vocations is that of prayer, according to the precept of the Lord: "The harvest indeed is abundant, but the laborers are few. Pray therefore the Lord of the harvest to send forth laborers into his harvest."(21) It is clearly indicated in these words of our divine Redeemer that the primary source of the priestly vocation is God himself, in His free and merciful will. Hence He said to His apostles: "You have not chosen me, but I have chosen you, and have appointed you that you should go and bear fruit, and that your fruit should remain."(22)
And St. Paul, while exalting the priesthood of Jesus above that of the Old Covenant, observed that every legitimate priest, being by nature a mediator between God and men, depends mainly on the divine benevolence: "For every high priest taken from among men is appointed for men in the things pertaining to God...And no man takes the honor to himself; he takes it who is called by God, as Aaron was one."(23)
How excellent, therefore, and a free gift is the calling to participate in the priesthood of Jesus Christ, of whom the same Apostle writes: "Christ did not glorify himself with the high priesthood...and when perfected, he became to all who obey Him the cause of eternal salvation, called by God a high priest according to the order of Melchisedech."(24)
Therefore with good reason St. John Chrysostom writes in his valuable treatise De sacerdotio: "Although the priesthood is exercised on earth it rightfully belongs to the celestial realm. For it was no man, nor angel, nor archangel, nor any other created power that arranged this function, but the Holy Spirit himself, and it was He, too, that inspired men to seek the ministry of angels."(25)
But when discussing this divine call to the priesthood (to which no one can claim any right) it is worth recalling that it concerns not only the spiritual faculties of the chosen one—his intelligence and free will—but involves also his sensitive faculties and even his very body. For the whole person must be fitted for the task of carrying out, in an efficient and worthy manner, the arduous duties of the sacred ministry, a ministry which often demands renunciation and sacrifice, sometimes even of one's own life after the example of the Good Shepherd, Jesus Christ.
We must not, however, imagine that God would call to the priesthood boys or young men who, insufficiently endowed in mind or heart, or because of obvious psychopathic weaknesses or serious organic defects, would afterwards be unable to carry out properly their various duties, or fulfill the obligations involved in the ecclesiastical life.
On the contrary it is comforting to hold the Angelic Doctor's doctrine that the Apostle's words about the first preachers of the Gospel can be applied equally to every one who is called to the priesthood. These are the words of St. Thomas: "Those whom God chooses for some task he so prepares and disposes that they may be found suitable for the task for which they are chosen in accordance with the words of the Second Epistle to the Corinthians,3, 6: `He also it is who has made us fit ministers of the new covenant."(26)
Full text of letter here: Vatican Website - Apostolic Letter 'Summai Dei Verbum' Pope Paul VI