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Where, when did Christ ever say that Rome would always have the Faith?

St. Robert Bellarmine
Lib. iv. De Rom. Pont. c. 4. sect. 1
Second Proposition: Not only the Roman Pontiff is unable to err in faith, but also the particular Roman Church. However it must be observed in this place, that the firmness of the Roman Church in the faith ought to be received in one sense, and the firmness of the pontiff in another: for the pontiff cannot err by judicial error: that is, when he judges and defines a question of faith. But the Roman Church, that is, the Roman people and clergy, cannot err by personal error, so that all together err, and no faithful would be in the Roman Church, adhering to the pontiff. For although each one individually can err, however it cannot happen that all together would err, and the whole Roman Church become apostate.
Lib. iv. De Rom. Pont. c. 4. sect. 5
For the authorities cited ... Agatho and Nicholas, popes and confessors ... assert that not only the pontiff, but also the Roman Church, is unable to err ... Another testimony is that of Pope Sixtus IV, who first through the Complutensian Synod, then also by himself, condemned (damnavit) the articles of Peter of Osma, of which articles one was, "The Church of the City of Rome can err". And although these statements seem to owe to be understood especially by reason of the pontiff, since however the Roman Church is not only the Pontiff, but the Pontiff and the people, so when the Fathers, or the Pontiffs, say that the Roman Church cannot err, they want to say; "In the Roman Church there will always be a Bishop teaching Catholicly and a people believing Catholicly."

First Vatican Council, Dogmatic Constitution on the Church
So the fathers of the fourth Council of Constantinople, following the footsteps of their predecessors, published this solemn profession of faith: The first condition of salvation is to maintain the rule of the true faith. And since that saying of our lord Jesus Christ, You are Peter, and upon this rock I will build my Church, cannot fail of its effect, the words spoken are confirmed by their consequences. For in the Apostolic See the Catholic religion has always been preserved unblemished, and sacred doctrine been held in honor. Since it is our earnest desire to be in no way separated from this faith and doctrine, we hope that we may deserve to remain in that one communion which the Apostolic See preaches, for in it is the whole and true strength of the Christian religion.

Funny that you quote St. Robert Bellarmine, who also said the following:

"Therefore, the true opinion is the fifth, according to which the Pope who is manifestly a heretic ceases by himself to be Pope and head, in the same way as he ceases to be a Christian and a member of the body of the Church; and for this reason he can be judged and punished by the Church. This is the opinion of all the ancient Fathers, who teach that manifest heretics immediately lose all jurisdiction, and outstandingly that of St. Cyprian (lib. 4, epist. 2) who speaks as follows of Novatian, who was Pope [antipope] in the schism which occurred during the pontificate of St. Cornelius: "He would not be able to retain the episcopate, and, if he was made bishop before, he separated himself from the body of those who were, like him, bishops, and from the unity of the Church."

Clearly this doctor of the Church erred. You should submit a petition to Rome to revoke his canonization, which was obviously in error.