[ultima ratio]

Novelties can't, by definition, be infallible. Here is Vatican I on this very subject:

"For the Holy Spirit was NOT promised to the Successors of Peter THAT by His revelation THEY MIGHT DISCLOSE NEW DOCTRINE, BUT THAT by His help THEY MIGHT GUARD the revelation TRANSMITTED through the apostles and the deposit of faith."

Popes can't invent religion, they can only transmit what they've received.

[gbcdoj]

My point was merely that the act of imposing the doctrine would prove it wasn't a "novelty".

But truths proposed for definitive belief by the Church can also be proposed by the canonical power, in which case they are believed with ecclesiastical faith and not divine faith. In the case of some such truths, they are not contained in the deposit of faith even implicitly, but they are necessary for guarding and expounding the deposit of faith. Such truths include the solemn canonizations of saints and the condemnations of Quanta Cura - and those who deny them separate themselves from the Church.