Leo XIII taught heresy while fraudulently posing as pope
Leo XIII Bestowed the Supreme Order of Christ on a Heretic
Anecdotal evidence against Leo XIII
Heretical Prayers of Antipope Leo XIII
As Catholics, we recognize that the seat of St. Peter has been vacant for some time (in fact since Pope Pius IX, the last valid Catholic pope), because we are very familiar with the Bull of Pope Paul IV, in which he states that any cleric found to have deviated from the Faith through heresy or schism, or even found to have provoked others to heresy or schism, PRIOR to his election to office, never actually held office at all, due to his sins against the Faith, which render him unfit for ecclesiastical office.
(i) each and every member of the following categories - Bishops, Archbishops, Patriarchs, Primates, Cardinals, Legates, Counts, Barons, Marquises, Dukes, Kings and Emperors - who:
(b) in the future also shall [so] deviate, or fall into heresy, or incur schism, or provoke or commit either or both of these, or shall be detected or shall confess to have, or shall be convicted of having [so] deviated, or fallen into heresy, or incurred schism, or provoked or committed either or both of these;"
6. In addition, [by this Our Constitution, which is to remain valid in perpetuity We enact, determine, decree and define:] that if ever at any time it shall appear that any Bishop, even if he be acting as an Archbishop, Patriarch or Primate; or any Cardinal of the aforesaid Roman Church, or, as has already been mentioned, any legate, or even the Roman Pontiff, prior to his promotion or his elevation as Cardinal or Roman Pontiff, has deviated from the Catholic Faith or fallen into some heresy: (i) the promotion or elevation, even if it shall have been uncontested and by the unanimous assent of all the Cardinals, shall be null, void and worthless;
(ii) it shall not be possible for it to acquire validity (nor for it to be said that it has thus acquired validity) through the acceptance of the office, of consecration, of subsequent authority, nor through possession of administration, nor through the putative enthronement of a Roman Pontiff, or Veneration, or obedience accorded to such by all, nor through the lapse of any period of time in the foregoing situation;
(iii) it shall not be held as partially legitimate in any way;
(iv) to any so promoted to be Bishops, or Archbishops, or Patriarchs, or Primates or elevated as Cardinals, or as Roman Pontiff, no authority shall have been granted, nor shall it be considered to have been so granted either in the spiritual or the temporal domain;
(v) each and all of their words, deeds, actions and enactments, howsoever made, and anything whatsoever to which these may give rise, shall be without force and shall grant no stability whatsoever nor any right to anyone;
(vi) those thus promoted or elevated shall be deprived automatically, and without need for any further declaration, of all dignity, position, honour, title, authority, office and power.
It is borderline blasphemy to quote an enemy of God and intone that his words are like the words of the Holy Ghost! To show you the kind of man that Pecci was quoting, here is a famous quote of Linnaeus: "It is not pleasing that I place Man among the primates, but man is intimately familiar with himself. Let's not quibble over words. It will be the same to me whatever name we use. But I request from you and from the whole world the generic difference between Man and Simian, and this from the principles of Natural History. I certainly know of none. If only someone might tell me just one! If I called man a simian or vice versa I would bring together all the theologians against me. Perhaps I ought to, in accordance with the law of the discipline [of Natural History]."
As if this was not enough, there is yet ANOTHER source showing Pecci to have been heretical before he even pretended to be pope, and this in a book that HE HIMSELF APPROVED!
"Cardinal" Pecci, Feb. 12, 1860: "let us limit ourselves to the consideration of the close connection which the spoliation of the Papal temporal power has with the interests of Catholic doctrine, with the mischievous results sure to follow for the Catholic religion. ... It is false that any Catholic holds the temporal dominion to be a dogma of his faith; such an assertion can only have come from the ignorance or the wickedness of the enemies of the Church." - S. Low, Marston, Searle & Rivington, 1887, Page 201.
WRONG Pseudo-Cardinal Pecci! Here is the very declaration of the dogma that you have denied:
Pope Boniface VIII, Unam Sanctam, 1302, ex cathedra: "We are informed by the texts of the gospels that in this Church and in its power are two swords; namely, the spiritual and the temporal. For when the Apostles say: 'Behold, here are two swords' [Lk 22:38] ... Certainly the one who denies that the temporal sword is in the power of Peter has not listened well to the word of the Lord commanding: 'Put up thy sword into thy scabbard' [Mt 26:52]. Both, therefore, are in the power of the Church, that is to say, the spiritual and the material sword, but the former is to be administered for the Church but the latter by the Church; the former in the hands of the priest; the latter by the hands of kings and soldiers, but at the will and sufferance of the priest. However, one sword ought to be subordinated to the other and temporal authority, subjected to spiritual power. ... For with truth as our witness, it belongs to spiritual power to establish the terrestrial power and to pass judgement if it has not been good. ... Therefore whoever resists this power thus ordained by God, resists the ordinance of God [Rom 13:2], unless he invent like Manicheus two beginnings, which is false and judged by us heretical, since according to the testimony of Moses, it is not in the beginnings but in the beginning that God created heaven and earth [Gen 1:1]. Furthermore, we declare, we proclaim, we define that it is absolutely necessary for salvation that every human creature be subject to the Roman Pontiff."