[The] Essential continuity in Christianity:  continuity of thought about the final significance of Jesus, continuity of a certain consciousness about history, continuity in the use of Scriptures, of bread and wine, of water.  But… these continuities are cloaked with such heavy veils belonging to their environment that Christians of different times and places must often be unrecognisable to others, or indeed themselves, as manifestations of a single phenomenon.

–  Andrew F. Walls, The Missionary Movement in Christian History:  Studies in the Transmission of Faith, 1996