All Christians, baptized in the name of the Father, Son and Holy Spirit are members of the Body of Christ, which is the Church, as we are told by Paul in Colossians.
One’s baptism cannot be undone.
As for Catholics who are actually Protestants, there is a misconception by them and many Protestants, that just because one attends Mass and receives the Eucharist, one is Catholic.
But, that is wrong.
One receives because one is Catholic. As Justin Martyr said in his letter to Caesar circa 150AD,
And this food is called among us Eukaristia [the Eucharist], of which no one is allowed to partake but the man who believes that the things which we teach are true, and who has been washed with the washing that is for the remission of sins, and unto regeneration, and who is so living as Christ has enjoined.
He is telling us here that the Eucharist is for those who believe what “we” teach are true. If one is receiving the Eucharist, then they are not Catholic in there beliefs and lives, they are in fact, Protestant and should not be receiving the Eucharist.
There are two antithetical paradigms in play on these threads that are argued over in the context of any Catholic versus Protestant discussion. These are individualism and communion.
Those who regard their individual interpretation of the Bible as the final authority are practitioners of individualism. While it is a desirable political philosophy individualism is contrary to the call to Christian unity.
On the other hand, when Catholics are in communion we accept the voice of the Church as the supreme authority, and therefore reject outright the principle of religious individualism. Attempted insults by non-Catholics that Catholics are not allowed to think for themselves are only an expression of an absence of Communion. Pray for them.
Peace be with you.
Just because you want to lump all people who are Christians, but not Roman Catholic, into a bucket labeled "Protestant" doesn't mean they are. By that same token, you shouldn't dump all your pseudo-Catholics into that same bucket either. I highly doubt many Catholic priests would even fall into line with everything that is deemed "essential" Catholic beliefs in faith and morals.
I appreciate that you admit that the Body of Christ IS all believers in Christ who have been baptized into Him. That is certainly a start in moving away from the exclusivity claimed by the Roman Catholic Church to that body. One important point, too, Justin Martyr was around in mid-second century, which places him before there even WAS a Roman Catholic Church having dominion over the Christian world. At his time, the truths taught in Scripture that the Apostles either wrote or authorized as Scripture were the rule of faith and were the source of "the things which we teach are true". Many of the doctrines taught today by the Roman Catholic Church were unheard of in Justin Martyr's time.