[small voice in the wilderness]

" In that which the faithful hold in common to be the true Catholic faith, they are infallible and "cannot err in matters of belief".- Second Vatican Council.

THIS is how they "know".

[metmom]

Hmm, the Catholic church then decided itself that it is infallible in its interpretation of Scripture, based on what?

Not Scripture for sure. There’s no Scripture to support that.

It must therefore be the tradition that they claim is on equal footing with the written word.

They have the whole thing sewed up top to bottom, all based on their say so.

How very convenient.