It is, I think, quite unfortunate that priests are no longer required to take this oath. In my opinion, all priests and Catholic teachers should be required to take it. The errors of Modernism have very badly infected the Church today, and are being taught in Catholic schools and preached from Catholic pulpits worldwide. Pope Pius X, near the beginning of the 20th century, masterfully showed in his encyclical Pascendi Dominici Gregis how Modernism is absolutely incompatible with the Catholic faith. He then required priests to swear the following oath, which also demonstrates very well the errors of modernism, and the truths in contradicts. [I will likely be posting more about Modernism in the future on this blog.]
I, [name], firmly embrace and accept all and everything that has been defined, affirmed, and declared by the unerring magisterium of the Church, especially those chief doctrines which are directly opposed to the errors of this time. And first, I profess that God, the beginning and end of all things, can be certainly known and thus can also be demonstrated by the natural light of reason "by the things that are made" [cf. Rom. 1:20], that is, by the visible works of creation, as the cause by the effects. Secondly, I admit and recognize the external arguments of revelation, that is, divine facts, and especially miracles and prophecies, as very certain signs of the divine origin of the Christian religion; and I hold that these same arguments have been especially accommodated to the intelligence of all ages and men, even of these times. Thirdly, likewise, with a firm faith I believe that the Church, guardian and mistress of the revealed word, was instituted proximately and directly by the true and historical Christ Himself, while he sojourned among us, and that the same was built upon Peter, the chief of the apostolic hierarchy, and his successors until the end of time. Fourthly, I accept sincerely the doctrine of faith transmitted from the apostles through the orthodox fathers, always in the same sense and interpretation, even to us; and so I reject the heretical invention of the evolution of dogmas, passing from one meaning to another, different from that which the Church first had; and likewise I reject all error whereby a philosophic fiction is substituted for the divine deposit, given over to the Spouse of Christ and to be guarded faithfully by her, or a creation of the human conscience formed gradually by the efforts of men and to be perfected by indefinite progress in the future. Fifthly, I hold most certainly and profess sincerely that faith is not a blind religious feeling bursting forth from the recesses of the subconscious, unformed morally under the pressure of the heart and the impulse of the will, but the true assent of the intellect to the truth received extrinsically ex auditu, whereby we believe that what has been said, attested, and revealed by the personal God, our Creator and Lord, to be true on account of the authority of God the highest truth.
I also subject myself with the reverence which is proper, and I adhere with my whole soul to all the condemnations, declarations, and prescriptions which are contained in the Encyclical letter, "Pascendi" and in the Decree, "Lamentabili", especially on that which is called the history of dogma. In the same manner I disapprove the error of those who affirm that the faith proposed by the Church can be in conflict with history, and that Catholic dogmas, in the sense in which they are now understood, cannot be reconciled with the more authentic origins of the Catholic religion.--I also condemn and reject the opinion of those who say that the more erudite Christian puts on a dual personality, one of the believer, the other of the historian, as if it were permitted the historian to hold what is in contradiction to the faith of the believer; or to establish premises from which it follows that dogmas are either false or doubtful, provided they are not directly denied.--I disapprove likewise that method of studying and interpreting Sacred Scripture, which disregards the tradition of the Church, the analogy of faith, and the norms of the Apostolic See, and adheres to the fictions of the rationalists, and no less freely than boldly adopts textual criticism as the only and supreme rule.--Besides I reject the opinion of those who hold that to present the historical and theological disciplines the teacher or the writer on these subjects must first divest himself of previously conceived opinion either on the supernatural origin of Catholic tradition, or on the aid promised by God for the perpetual preservation of every revealed truth; then that the writings of the individual Fathers are to be interpreted only by the principles of science, setting aside all divine authority, and by that freedom of judgment with which any profane document is customarily investigated. Finally, in short, I profess to be utterly free of the error according to which the modernists hold that there is nothing divine in the sacred tradition; or, what is far worse, admit this in the pantheistic sense, so that nothing remains but the bare and simple fact to be assimilated with the common facts of history, namely, of men by their industry, skill, and genius continuing through subsequent ages the school inaugurated by Christ and His disciples. So I retain most firmly the faith of the Fathers, and shall retain it until the final breath of life, regarding the certain gift of truth, which is, was, and will be always in the succession of the episcopacy from the apostles, not so that what may seem better and more fitting according to each one's period of culture may be held, but so that the absolute and immutable truth preached by the apostles from the beginning may never be believed otherwise, may never be understood otherwise.
All these things I promise that I shall faithfully, completely, and sincerely keep and inviolably watch, never deviating from them in word and writing either while teaching or in any other pursuit. So I promise, so I swear, so help me God.