[Mrs. Don-o]

"On what basis could a Catholic refuse to follow a decree by “the Vicar of Christ”?"

No one is ever obliged to obey a sinful command, by anybody. But I think you already know that.

[count-your-change]

I also know that that while correct doesn’t provide a basis for differentiating the sinful command from a Pope from the lawful which was what I asked, “the basis”.

If you say (and I agree):

“No one is ever obliged to obey a sinful command, by anybody. But I think you already know that.”

Then there must be some criteria available to “true believer” so as not to find himself being led into sin or being a rebel or some such.

Yes, it is a philosophical point but at the same time a practical one too.