You cite Pope Boniface's bull Unam Sanctam, but this is not a helpful example, since the teaching of this papal bull was never fully received into the teaching of the Catholic Church. As you know, the Catholic Church does not teach that the Bishop of Rome is infallible in all of his utterances. Not only is Unam Sanctam not recognized by most Catholic theologians as containing a binding dogmatic definition, but it is studied precisely as an example of a papal encyclical that does not fulfill the conditions of infallibility.
OK, it is easy for Catholic apologists to poke fun at UNAM SANCTAM and CANTATE DOMINO and say their teaching is irregular and that Peter had a momentary lapse of sanity.
But - wait for it!
- here is the Quote Mine of papal teachings through the centuries which shows that UNAM SANCTAM and CANTATE DOMINO are in fact the rock solid teaching of the Vicar of Christ
Fourth Lateran Council (1215): "There is but one universal Church of the faithful, outside which no one at all is saved."
Pope Boniface VIII, Bull "Unam sanctam" (1302): "We are compelled in virtue of our faith to believe and maintain that there is only one holy Catholic Church, and that one is apostolic. This we firmly believe and profess without qualification. Outside this Church there is no salvation and no remission of sins, the Spouse in the Canticle proclaiming: 'One is my dove, my perfect one. One is she of her mother, the chosen of her that bore her' (Canticle of Canticles 6:
; which represents the one mystical body whose head is Christ, of Christ indeed, as God. And in this, 'one Lord, one faith, one baptism' (Ephesians 4:5). Certainly Noah had one ark at the time of the flood, prefiguring one Church which perfect to one cubit having one ruler and guide, namely Noah, outside of which we read all living things were destroyed… We declare, say, define, and pronounce that it is absolutely necessary for the salvation of every human creature to be subject to the Roman Pontiff."
Pope Eugene IV, "Cantate Domino" (1441): "The most Holy Roman Church firmly believes, professes and preaches that none of those existing outside the Catholic Church, not only pagans, but also Jews and heretics and schismatics, can have a share in life eternal; but that they will go into the "eternal fire which was prepared for the devil and his angels" (Matthew 25:41), unless before death they are joined with Her; and that so important is the unity of this ecclesiastical body that only those remaining within this unity can profit by the sacraments of the Church unto salvation, and they alone can receive an eternal recompense for their fasts, their almsgivings, their other works of Christian piety and the duties of a Christian soldier. No one, let his almsgiving be as great as it may, no one, even if he pour out his blood for the Name of Christ, can be saved, unless he remain within the bosom and the unity of the Catholic Church."
Pope Sylvester II, Profession of Faith, June AD 991: "I believe that in Baptism all sins are forgiven, that one which was committed originally as much as those which are voluntarily committed, and I profess that outside the Catholic Church no one is saved."
Pope Innocent III (1198–1216), Profession of Faith prescribed for the Waldensians: "With our hearts we believe and with our lips we confess but one Church, not that of the heretics, but the Holy Roman Catholic and Apostolic Church, outside which we believe that no one is saved" (Denzinger 792).
Pope Clement VI, Letter "Super Quibusdam" (to Consolator the Catholicos of Armenia), September 20, 1351: "In the second place, we ask whether you and the Armenians obedient to you believe that no man of the wayfarers outside of the faith of this Church, and outside the obedience of the Pope of Rome, can finally be saved… In the ninth place, if you have believed and do believe that all who have raised themselves against the faith of the Roman Church and have died in final impenitence have been damned and have descended to the eternal punishments of hell."
Pope Leo XII (1823–1829), Encyclical "Ubi Primum": "It is impossible for the most true God, who is Truth Itself, the best, the wisest Provider, and rewarder of good men, to approve all sects who profess false teachings which are often inconsistent with one another and contradictory, and to confer eternal rewards on their members. For we have a surer word of the prophet, and in writing to you We speak wisdom among the perfect; not the wisdom of this world but the wisdom of God in a mystery. By it we are taught, and by divine faith we hold, one Lord, one faith, one baptism, and that no other name under heaven is given to men except the name of Jesus Christ of Nazareth in which we must be saved. This is why we profess that there is no salvation outside the Church… For the Church is the pillar and ground of the truth. With reference to those words Augustine says: 'If any man be outside the Church he will be excluded from the number of sons, and will not have God for Father since he has not the Church for mother.'"
Pope Gregory XVI (1831–1846), Encyclical "Summo Jugiter Studio" (on Mixed marriages), 5-6, May 27, 1832: "You know how zealously Our predecessors taught that very article of faith which these dare to deny, namely the necessity of the Catholic faith and of unity for salvation. The words of that celebrated disciple of the Apostles, martyred Saint Ignatius, in his letter to the Philadelphians are relevant to this matter: 'Be not deceived, my brother; if anyone follows a schismatic, he will not attain the inheritance of the kingdom of God.' Moreover, Saint Augustine and the other African bishops who met in the Council of Cirta in the year 412 explained the same thing at greater length: 'Whoever has separated himself from the Catholic Church, no matter how laudably he lives, will not have eternal life, but has earned the anger of God because of this one crime: that he abandoned his union with Christ' (Epsitle 141). Omitting other appropriate passages which are almost numberless in the writings of the Fathers, We shall praise Saint Gregory the Great, who expressly testifies that this is indeed the teaching of the Catholic Church. He says: 'The holy universal Church teaches that it is not possible to worship God truly except in her and asserts that all who are outside of her will not be saved' (Moral. in Job, 16.5). Official acts of the Church proclaim the same dogma. Thus, in the decree on faith which Innocent III published with the synod of the Lateran IV, these things are written: 'There is one universal Church of the faithful outside of which no one at all is saved.' Finally, the same dogma is expressly mentioned in the profession of faith proposed by the Apostolic See, not only that which all Latin churches use (Creed of the Council of Trent), but also that which the Greek Orthodox Church uses (cf. Gregory XIII, Profession 'Sanctissimus') and that which other Eastern Catholics use (cf. Benedict XIV, Profession 'Nuper ad Nos')… We are so concerned about this serious and well known dogma, which has been attacked with such remarkable audacity, that We could not restrain Our pen from reinforcing this truth with many testimonies."
Pope Pius IX (1846–1878), Allocution "Singulari Quadem", December 9, 1854: "Not without sorrow we have learned that another error, no less destructive, has taken possession of some parts of the Catholic world, and has taken up its abode in the souls of many Catholics who think that one should have good hope of the eternal salvation of all those who have never lived in the true Church of Christ. Therefore, they are wont to ask very often what will be the lot and condition of those who have not submitted in any way to the Catholic faith, and, by bringing forward most vain reasons, they make a response favorable to their false opinion. Far be it from Us, Venerable Brethren, to presume on the limits of the divine mercy which is infinite; far from Us, to wish to scrutinize the hidden counsel and "judgements of God" which are "a great abyss" (Ps. 35.7) and cannot be penetrated by human thought. But, as is Our Apostolic Duty, we wish your episcopal solicitude and vigilance to be aroused, so that you will strive as much as you can to drive form the mind of men that impious and equally fatal opinion, namely, that the way of eternal salvation can be found in any religion whatsoever. May you demonstrate with skill and learning in which you excel, to the people entrusted to your care that the dogmas of the Catholic faith are in no wise opposed to divine mercy and justice.
"For, it must be held by faith that outside the Apostolic Roman Church, no one can be saved; that this is the only ark of salvation; that he who shall not have entered therein will perish in the flood; but, on the other hand, it is necessary to hold for certain that they who labor in ignorance of the true religion, if this ignorance is invincible, will not be held guilty of this in the eyes of God. Now, in truth, who would arrogate so much to himself as to mark the limits of such an ignorance, because of the nature and variety of peoples, regions, innate dispositions, and of so many other things? For, in truth, when released from these corporeal chains 'we shall see God as He is' (1 John 3.2), we shall understand perfectly by how close and beautiful a bond divine mercy and justice are united; but as long as we are on earth, weighed down by this mortal mass which blunts the soul, let us hold most firmly that, in accordance with Catholic teaching, there is "one God, one faith, one baptism" (Eph. 4.5); it is unlawful to proceed further in inquiry.
"But, just as the way of charity demands, let us pour forth continual prayers that all nations everywhere may be converted to Christ; and let us be devoted to the common salvation of men in proportion to our strength, 'for the hand of the Lord is not shortened' (Isa. 9.1) and the gifts of heavenly grace will not be wanting to those who sincerely wish and ask to be refreshed by this light."
Pope Pius IX (1846–1878), Encyclical "Singulari Quidem" March 17, 1856): "Teach that just as there is only one God, one Christ, one Holy Spirit, so there is also only one truth which is divinely revealed. There is only one divine faith which is the beginning of salvation for mankind and the basis of all justification, the faith by which the just person lives and without which it is impossible to please God and come to the community of His children (Romans 1; Hebrews 11; Council of Trent, Session 6, Chapter
. There is only one true, holy, Catholic Church, which is the Apostolic Roman Church. There is only one See founded on Peter by the word of the Lord (St. Cyprian, Epistle 43), outside of which we cannot find either true faith or eternal salvation. He who does not have the Church for a mother cannot have God for a father, and whoever abandons the See of Peter on which the Church is established trusts falsely that he is in the Church (ibid, On the Unity of the Catholic Church). ... Outside of the Church, nobody can hope for life or salvation unless he is excused through ignorance beyond his control."
Pope Pius IX (1846–1878), Encyclical "Quanto conficiamur moerore", August 10, 1863: "And here, beloved Sons and Venerable Brothers, We should mention again and censure a very grave error in which some Catholics are unhappily engaged, who believe that men living in error, and separated from the true faith and from Catholic unity, can attain eternal life. Indeed, this is certainly quite contrary to Catholic teaching. It is known to Us and to you that they who labor in invincible ignorance of our most holy religion and who, zealously keeping the natural law and its precepts engraved in the hearts of all by God, and being ready to obey God, live an honest and upright life, can, by the operating power of divine light and grace, attain eternal life, since God who clearly beholds, searches, and knows the minds, souls, thoughts, and habits of all men, because of His great goodness and mercy, will by no means suffer anyone to be punished with eternal torment who has not the guilt of deliberate sin. But, the Catholic dogma that no one can be saved outside the Catholic Church is well-known; and also that those who are obstinate toward the authority and definitions of the same Church, and who persistently separate themselves from the unity of the Church, and from the Roman Pontiff, the successor of Peter, to whom 'the guardianship of the vine has been entrusted by the Savior,' (Council of Chalcedon, Letter to Pope Leo I) cannot obtain eternal salvation. The words of Christ are clear enough: 'And if he will not hear the Church, let him be to thee as the heathen and publican' (Matthew 18:17); 'He that heareth you, heareth Me; and he that dispeth you, despiseth Me; and he that dispiseth Me, despiseth Him that sent Me' (Luke 10:16); 'He that believeth not shall be condemned' (Mark 16:16); 'He that doth not believe, is already judged" (John 3:18); 'He that is not with Me, is against Me; and he that gathereth not with Me, scattereth' (Luke 11:23). The Apostle Paul says that such persons are 'perverted and self-condemned' (Titus 3:11); the Prince of the Apostles calls the 'false prophets… who shall bring in sects of perdition, and deny the Lord who bought them: bringing upon themselves swift destruction' (2 Peter 2:1)."
Pope Pius IX The "Syllabus of Errors", attached to Encyclical Quanta Cura, 1864: [The following are prescribed errors:] "16. Men can, in the cult of any religion, find the way of eternal salvation and attain eternal salvation. - Encyclical Qui pluribus, November 9, 1846.
"17. One ought to at least have good hope for the eternal salvation of all those who in no way dwell in the true Church of Christ. - Encyclical Quanto conficiamur moerore, August 10, 1863, etc."
Pope Leo XIII (1878–1903), Encyclical Annum Ingressi Sumus: "This is our last lesson to you; receive it, engrave it in your minds, all of you: by God's commandment salvation is to be found nowhere but in the Church."
idem, Encyclical "Sapientiae Christianae": "He scatters and gathers not who gathers not with the Church and with Jesus Christ, and all who fight not jointly with Him and with the Church are in very truth contending against God."
Pope St. Pius X (1903–1914), Encyclical "Jucunda Sane": "It is our duty to recall to everyone great and small, as the Holy Pontiff Gregory did in ages past, the absolute necessity which is ours, to have recourse to this Church to effect our eternal salvation."
Pope Benedict XV (1914–1922), Encyclical "Ad Beatissimi Apostolorum": "Such is the nature of the Catholic faith that it does not admit of more or less, but must be held as a whole, or as a whole rejected: This is the Catholic faith, which unless a man believe faithfully and firmly, he cannot be saved."
Pope Pius XI (1922–1939), Encyclical "Mortalium Animos": "The Catholic Church alone is keeping the true worship. This is the font of truth, this is the house of faith, this is the temple of God; if any man enter not here, or if any man go forth from it, he is a stranger to the hope of life and salvation… Furthermore, in this one Church of Christ, no man can be or remain who does not accept, recognize and obey the authority and supremacy of Peter and his legitimate successors."
Pope Pius XII (1939–1958), Encyclical "Humani Generis", August 12, 1950: "Some reduce to a meaningless formula the necessity of belonging to the true Church in order to gain eternal salvation."
Pope Pius XII (1939–1958), Allocution to the Gregorian University (17 October 1953): "By divine mandate the interpreter and guardian of the Scriptures, and the depository of Sacred Tradition living within her, the Church alone is the entrance to salvation: She alone, by herself, and under the protection and guidance of the Holy Spirit, is the source of truth."