And, on a lay person individual level, I have answered your question: use Scripture, under the guidance of tradition and reason. It’s a case-by-case basis.
Again, you need to give a specific example of a papal decree asking people to commit a mortal sin, if you want more specifics than the above. (Say what you will about them, but believing in the Assumption and the Immaculate Conception certainly does not detract from our state of grace or salvation.)
And, on a lay person individual level, I have answered your question: use Scripture, under the guidance of tradition and reason. It’s a case-by-case basis.
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The reason that PFRotestants tend to get annoyed with that answer is that when WE use Scripture we get yelled at and told that we have no right to interpret Scripture for ourselves.
But that's the issue....the Roman Catholic has to believe in these. If you don't that's the mortal sin. That you have to believe this for your salvation is the mortal sin.
And, on a lay person individual level, I have answered your question: use Scripture, under the guidance of tradition and reason. It’s a case-by-case basis.
To Luircin's point....the laity in Roman Catholicism cannot understand Scripture on their own. We have been told that too many times on these threads. They are dependent upon a priest to interpret the Scripture for them. And that is going to be a problem for even the priest as the Vatican has only dogmatically ruled on only no more than 14 verses.