Paul wrote this before the New Testament canon was finalized.
Paul also said, “stand firm and hold to the traditions that you were taught by us, either by *our* spoken word or by our letter”
and “If anyone thinks he is a prophet or spiritually gifted, let him acknowledge that what I am writing to you is the Lord’s command”
He also referred to "the church of the living God, which is *the pillar and foundation of the truth*."
Protestants also slaughtered thousands for their beliefs - not a great argument imo. persecution doesn't settle theological disputes.
