[ebb tide]

If by “they” he means that the individual believes that he possesses all the truth, then the individual has a problem/

Well, "they" implies more than one individual.

Bergoglio went on to state:

“You cannot cancel a whole religion because there is a group or many groups of fundamentalists at certain moments of history.”

I highly suspect Bergolio's definition of "fundamentalist" Catholics is directed towards those traditional, orthodox, pelagian, and rigid Catholics as a whole.

[Hieronymus]

Well, “they” implies more than one individual.

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Yes, but you can have multiple individuals each believing that the fulness of truth resides in himself alone. You can also have people believing that the fulness resides in some fragment of the church.

That said, I wouldn’t bet money on my out being the correct interpretation.