I look at it this way - it is possible that a Catholic also be a Christian despite their doctrine. The gospel is there, just obscured by paganism and tradition. If they read their bibles and read the gospel of Jesus Christ and choose Catholicism then I really have nothing to offer. They made a choice. Some will be saved and remain Catholic out of tradition. This is a sad truth, but they are no less Christian. Most will perish, but this probably could be said of any denomination.
What I have witnessed is Christians showing Catholics the gospel and their reply being "by whose interpretation". The answer is not by any interpretation but by God's words.
Christians disagree often on interpretation. But not on the gospel, not on praying to saints being blasphemy, not on praying to Mary, not on so many things inherent in Catholic paganism.
We all accept that the Bible has Authority, but it cannot be the only Authority, because it is at the point of interpretation in truth that the Bible expresses its full Authority or it is nullified by wrongful interpretations.
So it is the Authority of the Bible’s interpreter that unlocks the Bible’s full Authority.
Each Bible aloner says he is guided by The Holy Spirit in his textual criticism of scripture as if he is his own inspired oracle.
This is not how The Holy Spirit operates.
If The Holy Spirit operated like this, all Protestants would be monolithic and have total unity with each other going back to Luther, despite any human frailty or weakness or ignorance.
I would be totally convinced of the truth of the Protestant position of Bible alone if Protestantism was singular and monolithic in every interpretation and doctrine. That on reading the Bible alone everyone was brought together by The Holy Spirit in the same understandings, interpretations and doctrines.
It would be impossible to argue with, because it would be manifest everywhere and I would be sitting next to you in church on Sunday singing the same tune.
The Holy Spirit is The Spirit of Truth, He is not going to be interpreting conflicting doctrines to different people, it would be the exact same every single time.
Protestantism has the wrong understanding of how The Holy Spirit operates.
The deposit of the Faith laid down once by Christ and The Apostles and preached by The Spirit, was the Truth.
This Truth by necessity became Tradition handed down without alteration guarded by The Holy Spirit teacher to teacher.
The Tradition of The Holy Spirit or as Catholics call it Apostolic Tradition.
Remember what Jesus told the disciples how The Holy Spirit would operate.
“ and will remind you of all I have said to you “
So The Holy Spirit operates on the memory of what was spoken by the preceding teachers. Christ to the Apostles, the Apostles to the Fathers, and the Fathers down to us.
A singular recollection of the same truth and faith and understanding handed down the ages, guarded by The Holy Spirit.
Protestantisms personal oracle idea is a manifest disaster, each comes away whatever his human opinion interprets from the scripture. Traditions of men.