Excerpt from Dom Prosper Guéranger's The Liturgical Year - Trinity Sunday

For this end, we will unite with holy Church, who in her Office of Prime, recites on this Solemnity, as also on every Sunday not taken up by a Feast, the magnificent Symbol known as the Athanasian Creed. It gives us, in a summary of much majesty and precision, the doctrine of the holy Doctor, Saint Athanasius, regarding the mysteries of the Trinity and Incarnation.

It is a psalm or hymn of praise, of confession, and of profound, self-prostrating homage, parallel to the Canticles of the elect in heaven. It appeals to the imagination quite as much as to the intellect. It is the war-song of faith, with which we warn first ourselves, then each other, and then all those who are within its hearing, and the hearing of the Truth, who our God is, and how we must worship Him, and how vast our responsibility will be if we know what to believe, and yet believe not.

The Athanasian Creed

Quote:The safest guiding principle in the crucial question for the life of the Church regarding the validity of the papacy of Pope Francis, should be the prevailing practice in the history of the Church, with which were resolved cases of presumably invalid papal renunciations or elections. In this prevailing practice was shown the sensus perennis ecclesiae.

Quote:“It has mattered naught that Monsignor Georg Gänswein, in a declaration to LifeSiteNews on February 14, 2019, reaffirmed the validity of Benedict XVI’s renunciation of the Petrine Office, by stating that “there is only one Pope legitimately elected – and it is Francis.” By then the idea of a possible redefinition of the Papal ministry had been launched. Some people say that pope Benedict’s intention was that of maintaining the papacy, assuming the office capable of bifurcating in two; but this is a substantial error, since the monarchal and unitary nature of the Papacy is of divine right.

God alone judges intentions, whereas Canon Law merely limits itself in evaluating the outward behavior of the baptized. A well-known sentence of canon law affirms that «De internis non iudicat praetor»; a judge does not judge interior things. On the other hand, Canon 1526, § 1 of the new Code of Canon Law points out that: «Onus probandi incumbit ei qui asserit» (the onus of providing the proofs falls to the one alleging). There is a difference between a clue and a proof. The clue suggests the possibility of a fact, the proof demonstrates the certainty. Agatha Christie’s rule whereby three clues are a proof, is fine in literature but not in civil or ecclesiastical courts. Furthermore, if Pope Benedict is the legitimate Pope, what would happen if from one day to the next he should die, or instead, before he died, Pope Francis should pass away? Given the fact that many current cardinals were created by Pope Francis and none of the cardinal electors consider him an Anti-Pope, the apostolic succession would be interrupted, jeopardizing the visibility of the Church. The paradox is that to prove the invalidity of Benedict’s renunciation juridical sophisms are employed, but then to resolve the problem of Benedict’s or Francis’s succession, extra-canonical solutions ought to be used.” (The Unknowns at the End of a Pontificate, originally in Corrispondenza Romana, July 1, 2020)

Archbishop Lefebvre repeatedly said that the Conciliar Church was schismatic, specifically because of it's new rites, laws, worship, institutions, etc:

• “What could be clearer? We must henceforth obey and be faithful to the Conciliar Church, no longer to the Catholic Church. Right there is our whole problem: we are suspended a divinis by the Conciliar Church, the Conciliar Church, to which we have no wish to belong! That Conciliar Church is a schismatic church because it breaks with the Catholic Church that has always been. It has its new dogmas, its new priesthood, its new institutions, its new worship… The Church that affirms such errors is at once schismatic and heretical. This Conciliar Church is, therefore, not Catholic. To whatever extent Pope, Bishops, priests, or the faithful adhere to this new church, they separate themselves from the Catholic Church.” (Archbishop Lefebvre, Reflections on his suspension a divinis, July 29, 1976)
  

• "It is not we who are in schism but the Conciliar Church." (Homily preached at Lille, August 29, 1976)
  

• “It is impossible for Rome to remain indefinitely outside Tradition. It’s impossible… For the moment they are in rupture with their predecessors. This is impossible. They are no longer in the Catholic Church.” (Retreat Conference, September 4, 1987, Ecône)
  

• “The magisterium of today is not sufficient by itself to be called Catholic unless it is the transmission of the Deposit of Faith, that is, of Tradition. A new magisterium without roots in the past, and all the more if it is opposed to the magisterium of all times, can only be schismatic and heretical. The permanent will to annihilate Tradition is a suicidal will, which justifies, by its very existence, true and faithful Catholics when they make the decision necessary for the survival of the Church and the salvation of souls. Our Lady of Fatima, I am sure, blesses this final appeal in this 70th anniversary of her apparitions and messages. May you not be for a second time deaf to her appeal.” (Archbishop Lefebvre, July 8, 1987, Excerpt from the Letter of Archbishop Lefebvre to Cardinal Ratzinger)
  

• “Well, we are not of this religion. We do not accept this new religion. We are of the religion of all time; we are of the Catholic religion. We are not of this 'universal religion' as they call it today-this is not the Catholic religion any more. We are not of this Liberal, Modernist religion which has its own worship, its own priests, its own faith, its own catechisms, its own Bible, the 'ecumenical Bible' - these things we do not accept.” (Sermon, July 29, 1976)