“Shouldn’t Protestants be protesting in the streets, calling him an infidel and making death threats against him?”
Bible-believing Christians believe in ‘soul-liberty’ and freedom of conscience. They might argue strongly for what they hold, but they don’t persecute (e.g. make “death threats”) people who profess that their conscience leads them otherwise.
Romans 14:12 - So then every one shall give account OF HIMSELF to God. No Presbyterian (in this example) has to give account to God for the direction of the former Presbyterian minister who converted to Catholicism. The man will answer for Himself, and therefore no Presbyterian is going to threaten the man, or rampage in the streets either.
Presbyterians, as far as I am aware, do not have an Inquisition.