Psalm 109:4
Alleluia, alleluia. The Lord hath sworn, and He will not repent: Thou art a priest for ever according to the order of Melchisedech. Alleluia.
Haydock
Verse 4. Repent. Not that He can ever do so, or give way to error: but the sacred writer expresses himself thus, to give us the greatest security. C. — The order. Heb. dibrathi, “my order,” Melchisedech. The i has been perhaps designedly inserted, to render the argumentation in the epistle to the Hebrews of no weight, “as the force of the text sinks into just nothing.” Kennicott. — Prot. and Pagnin here abandon the Heb. But Montanus corrects the latter, and substitutes “upon my word,” which is more honest, as he deemed the Heb. text unerring, though here it be not so unquestionably, as the Almighty would thus address Melchisedech, unless that title be here given to Christ. S. Jerom takes no notice of my, no more than the apostle, &c. H. — This instance “may perhaps put all serious Christians upon deliberating—whether they should any longer maintain the absolute integrity of the present Hebrew text.” See Ps. xv. 10. Kennicott, Dis. i. p. 219. — Melchisedech. Christ is declared king and priest for ever, (W.) like Melchisedech, who united in his person both dignities, and presided not over a particular people, nor stood in need of any stated place. His succession is not recorded, and his sacrifice consisted of bread and wine; in all which respects he differed from the Levitical priest, and prefigured Christ, who is immolated under the same species throughout the world. Mal. i. 11. M. — We read in Scripture of three orders of priesthood: 1st, of kings, 2d, of the first-born, and 3d, of Aaron. Melchisedech, in quality of king, exercised the priestly office, as both functions were formerly united: and hence the word Cohen signifies both a temporal and spiritual prince. This light of nations ensured to his order a perpetual duration, whilst that of Aaron was to have an end. Thus Christ offered to his Father from all eternity the sacrifice of his obedience, and future sufferings; and in time, he presented that of his own life, which he continues to offer in the Catholic Church, (C.) by priests who are only his ministers. 1 Cor. vi. W. — The apostle does not specify the oblation of bread and wine, as it was unnecessary, the sacrifice of Christ on the cross having put an end to the sacrifices of the old law, which could only be offered by the children of Aaron, from whom he did not spring. This was enough for his purpose. But as Menchisedech offered bread and wine, Christ must also have done the same, to be of his order. S. Cyprian, and the other Fathers, with great unanimity observe, that the sacrifice of Melchisedech was a figure of that of Jesus Christ, in bread and wine; and of course (Bert.) our sacred mysteries must contain the substance. H. — By their application, Christ still pacifies his Father in behalf of sinners: so that the effects of his priesthood do not cease, as those of all the priests in the Old Testament did by their death. W.
Denzinger
938: The Institution of the Most Holy Sacrifice of the Mass
Council of Trent Session XXII (Sept. 17, 1562)
Since under the former Testament (as the Apostle Paul bears witness) there was no consummation because of the weakness of the Levitical priesthood, it was necessary (God the Father of mercies ordaining it thus) that another priest according to the order of Melchisedech [Gen. 14:18 ;Ps. 109:4;Heb. 7:11] arise, our Lord Jesus Christ, who could perfect [Heb. 10:14] all who were to be sanctified, and lead them to perfection. He, therefore, our God and Lord, though He was about to offer Himself once to God the Father upon the altar of the Cross by the mediation of death, so that He might accomplish an eternal redemption for them [edd.: illic,there], nevertheless, that His sacerdotal office might not come to an end with His death [Heb. 7:24, 27] at the Last Supper, on the night He was betrayed, so that He might leave to His beloved spouse the Church a visible sacrifice [can. 1] (as the nature of man demands), whereby that bloody sacrifice once to be completed on the Cross might be represented, and the memory of it remain even to the end of the world [1 Cor. 11:23 ff.] and its saving grace be applied to the remission of those sins which we daily commit, declaring Himself constituted “a priest forever according to the order of Melchisedech” Ps. 109:4; offered to God the Father His own body and blood under the species of bread and wine, and under the symbols of those same things gave to the apostles (whom He then constituted priests of the New Testament), so that they might partake, and He commanded them and their successors in the priesthood in these words to make offering: “Do this in commemoration of me, etc.” [Luke 22:19;1 Cor. 11:23], as the Catholic Church has always understood and taught [can. 2]. For, after He had celebrated the ancient feast of the Passover, which the multitude of the children of Israel sacrificed [Exod. 12:1 ff.] in memory of their exodus from Egypt, He instituted a new Passover, Himself to be immolated under visible signs by the Church through the priests, in memory of His own passage from this world to the Father, when by the shedding of His blood He redeemed us and “delivered us from the power of darkness and translated us into His kingdom [Col. 1:13].
2275: The Effects of the Order of the Priesthood
Pope Pius XI From the Encyclical "Ad catholic) sacerdotii," December 20, 1935
The minister of Christ is the priest; therefore, he is, as it were, the instrument of the divine Redeemer, that He may be able to continue through time His marvelous work which by its divine efficacy restored the entire society of men and brought it to a higher refinement. Rather, as we customarily say rightly and properly: “He is another Christ,” since he enacts His role according to these words: “As the Father has sent me, I also send you” [John 20:21]; and in the same way and through the voice of the angels his Master sings: “Glory to God in the highest,” and exhorts peace “to men of good will” [cf. Luke 2:14].… Such powers, conferred upon the special sacrament of the priesthood, since they become imprinted on his soul with the indelible character by which, like Him whose priesthood he shares, he becomes “a priest forever” [Ps. 109:4], are not fleeting and transitory, but stable and permanent. Even if through human frailty he lapse into errors and disgraces, yet he will never be able to delete from his soul this sacerdotal character. And besides, through the sacrament of orders the priest not only acquires the sacerdotal character, not only high powers, but he is also made greater by a new and special grace, and by special helps, through which indeed—if only he will faithfully comply, by his free and personal cooperation, with the divinely efficient power of these heavenly gifts, surely he will be able worthily and with no dejection of spirit to meet the arduous duties of his ministry.…
From holy retreats [of spiritual exercises] of this kind such usefulness can also at times flow forth, that one, who has entered “in sortem Domini” not at the call of Christ Himself but induced by his earthly motives, may be able “to stir up the grace of God” [cf. 2 Tim. 1:6]; for since he is now bound to Christ and the Church by an everlasting bond, he can accordingly do nothing but adopt the words of St. Bernard: “For the future make good your ways and your ambitions and make holy your ministry; if sanctity of life did not precede, at least let it follow.” The grace which is commonly given by God and is given in a special manner to him who accepts the sacrament of orders, will undoubtedly aid him, if he really desires it, no less for emending what in the beginning was planned wrongly by him, than for executing and taking care of the duties of his office.