One who abandons or withdraws from an established religion, opinion, custom, creed, etc.(noun)
One who seeks radical political or religious reform.(noun)
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Use "come-outer" in a sentence
"Like a majority of the members, I was a "come-outer" from another faith, in my case Southern Baptist."
"My own view is that the UUA exploits the newcomer/come-outer myth to hold onto its own highly centralized self-referrent power in exactly the same kind of small-church polity it condemns in our congregations."
"He would have nothing of worldly advantage to gain and much to lose as a "come-outer" from the Establishment."