Narthex is alive and well in English. Exonarthex is a recent borrowing into architectural nomenclature. The practices of the early Church that required an exonarthex were abandoned early, but the architectural bits lived on. This is demonstrated in modern terms by St. Peter's Square in Rome, which constitutes the grandest exonarthex of all; the chief officiant (i.e., the pope) comes out and reveals all the mysteries formerly celebrated within.
From exo- + narthex.