Interestingly, a Resistance priest (not a native English speaker) following this thread sends me this:
"I don't think Our Lady has ever said 'Rome will lose faith and become the Seat of the Antichrist', unless She is referring, like Viganò, to the city.
Even if the sentence is true, the interpretation that sedevacantists (and many non-sedevacantist traditionalists) make of it is far from the only conceivable one. The version of the secret that contains said sentence also mentions two other cities: Paris and Marseille: "Paris will be burned and Marseilles will be swallowed up." And all agree that Paris means the city, not the government of France, not the Archbishop of Paris, not his curia, not the diocese, etc.; and all agree that Marseilles means the city, not the Bishop of Marseilles, nor his curia, nor the diocese, nor the civil government of that city.Then, congruently, the word Rome must also be understood in a rather literal sense, as in the other two cases, meaning not the Holy See, but the population of the city when it says "will lose faith", and Rome as a physical place when it says "and will become the seat of the Antichrist" Thus, the most probable sense of the phrase would be that the inhabitants of Rome will lose faith and that the most important directing body of anti-Catholic action will have its seat in this city, such as a world command of Fɾҽҽmαsσɳɾყ. This interpretation seems very reasonable, since the devil, wanting to imitate the works of God, would place the seat of his vicar, who is the Antichrist, next to the seat of the Vicar of Christ."
But that this portion of the Secret is condemned in any case is indisputable.