Christianity didn't die. The bulk of it was superceded by Calvinist philosophy and the Inquisitions murdered most of the gnostics and copics and therefore their thoughts and knowledge.

But, there are a few branches surviving today which rejected Calvinism, Catholicism, etc and hold to the teachings of Christ. The Unitarians and Quakers come to mind.

I’m sure there are others in other countries but I cannot speak to them as I have no experience. Maybe someone else can add in?