I always chuckle when a Catholic who views scripture as secondary to the magesterium claim Protestants who view scripture as their only source don’t know scripture.
What authority to Protestants recognize outside of Scripture? Luther?
Protestants believe many things. Not all Protestants adhere even to Luther's doctrine of Sola Scriptura, i.e, The Church of England.
Not all Protestants even adhere to the doctrine of the Trinity, i.e., Oneness Pentecostals.
What single doctrine do they ALL adhere to?
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They reject the authority of the Catholic Church.
This is a very serious thing, since St. Paul calls the church, "the pillar and foundation of truth," and Jesus said, "if he will not listen to the church, treat him as a pagan or tax collector."