Synonyms for Ineffable (same or very similar meaning)
WordNet sense 1 (too sacred to be uttered):
unnameable, unspeakable, unutterableWordNet sense 2 (concerned with religion or religious purposes):
sacredWordNet sense 3 (defying expression or description):
indefinable, indescribable, untellableWordNet sense 4 (defying expression):
inexpressible, unexpressibleFrom the ODE community, based on WordNetadd/edit