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Who are these non-denominational Christians for two-thousand years?

Those born-again-of-the-Spirit believers in Lord Jesus Christ alone as their Savior from their sins and the second death, and those who escaped the clutches of religious and secular tyrants who tried to murder them or at least keep them imprisoned for life and isolated from other people.

Non-denominational Christians: The Remnant you've read about in the Holy Bible.

That's my answer (again) to your repeated question.