The Ever-new Tongue (In Tenga Bithnua), composed in Ireland in the ninth or tenth century, purports to reveal the mysteries of the creation, of the cosmos, and of the end of the world, as related by the soul of the apostle Philip speaking in the language of the angels. Drawing on a multitude of sources, both mainstream and heterodox, it reflects the richness of early Irish learning as well as the vitality of its author's imagination. The present volume is based on the full critical edition of The Ever-new Tongue, including detailed linguistic analysis and textual notes, which appeared in 2009 in the Corpus Christianorum, Series Apocryphorum (CCSA 16). The aim here is to offer to a broader readership a translation of the oldest (and most conservative) version of the text, preserved in the Book of Lismore, together with such other parts, fully updated, of the larger study as may be of interest to non-Celticists.