We have the promise of Christ that He would be with His Church until the end of time. That means the possibility of having a Pope who worships satan is equal to the possibility of a four sided triangle.
The opinion that a Pope can never lose the Faith is not something Catholics are required to believe.
If I were to accept that a Pope could teach error, or that a man who was not the authentic Successor of Blessed Peter could sit in the Chair of Blessed Peter, unchallenged by a true Successor of Blessed Peter in the Chair of Peter, then I would have to believe that Christ has indeed deserted His Church.
No, you would have to conclude that the man you considered to be Pope is not Pope.
There has never been a false claimant to the Chair of Blessed Peter who was unchallenged by the true Successor of Blessed Peter.
These Popes are not unchallenged nor are they universally accepted.
Infallibility isn't a tautology.
It doesn't mean "the Pope is infallible except when he's saying things with no basis in prior Tradition" - the sifting position.
It doesn't mean "what the Pope teaches is true by virtue of the fact the Pope teaches it" - that's the neo-Cath position.
What it means, is that we can be sure that a true Pope remains faithful and teaches the true religion and is its supreme authority.
But knowing this does not mean we throw away our own reason and accept absurd contradictions as being possible. It is because of the infallibility of true Popes that we know what the Church teaches and has always taught, and because of that, we
can recognize a false teacher and
judge his false teachings.