In Praeclara Summorum §6

⁶ No need to recall Alighieri's great reverence for the authority of the Catholic Church, the account in which he holds the power of the Roman Pontiff as the base of every law and institution of that Church. Hence the outspoken warning to Christians: You have the Old and the New Testament: the Pastor of the Church as Guide; Let that suffice for your salvation. He felt the troubles of the Church as his own, and while he deplored and condemned all rebellion against its Supreme Head he wrote as follows to the Italian Cardinals during the stay at Avignon: "To us who confess the same Father and Son, the same God and Man, the same Mother and Virgin; to us for whom and for whose salvation the message was given, after the triple Lovest thou Me? Feed My sacred sheepfold; to us, driven to mourn with Jeremias - but not over things to come but over things that are - for Rome - that Rome on which Christ, after all the old pomp and triumph, confirmed by word and work the empire of the world, and which Peter, too, and Paul the Apostle of the Nations consecrated with their very blood as Apostolic See - now widowed and desolate; to us it is as terrible grief to see this as to see the tragedy of heresy" ( Epist . VIII). For him the Roman Church is The Most Holy Mother, Bride of Him Crucified and to Peter, infallible judge of revealed truths, is owing perfect submission in matters of faith and morals. Hence, however much he may hold that the dignity of the Emperor is derived immediately from God, still he asserts that this truth "must not be understood so strictly as to mean that the Roman Prince is not subject to the Roman Pontiff in anything, because this mortal happiness is subjected in certain measure to immortal happiness" ( Mon . III, 16). Excellent and wise principle indeed which, if it were observed today as it ought to be, would bring to States abundant fruits of civil prosperity. But, it will be said, he inveighs with terrible bitterness against the Supreme Pontiffs of his times. True; but it was against those who differed from him in politics and he thought were on the side of those who had driven him from his country. One can feel for a man so beaten down by fortune, if with lacerated mind he breaks out sometimes into words of excessive blame, the more so that, to increase his feeling, false statements were being made by his political enemies ready, as always happens, to give an evil interpretation to everything. And indeed, since, through mortal infirmity, "by worldly dust even religious hearts must needs be soiled" (St. Leo M. S. IV de Quadrag), it cannot be denied that at that time there were matters on which the clergy might be reproved, and a mind as devoted to the Church as was that of Dante could not but feel disgust while we know, too, that reproof came also from men of conspicuous holiness. But, however he might inveigh, rightly or wrongly, against ecclesiastical personages, never did he fail in respect due to the Church and reverence for the "Supreme Keys"; and on the political side he laid down as rule for his views "the reverence which a good son should show towards his father, a dutiful son to his mother, to Christ, to the Church, to the Supreme Pastor, to all who profess the Christian religion, for the safeguarding of truth" ( Mon . III, 3).