Celtic Studies examines the languages, literatures and religious traditions of Celtic speaking peoples, ancient and medieval, asking what a mythological or ritual claim actually rests on in the surviving textual and archaeological record rather than in later reconstruction. The Bavarian scholar Johann Kaspar Zeuss founded the discipline with his Grammatica Celtica, published in 1853, a work built almost entirely from Old Irish glosses he had traced through continental libraries even though he never visited Ireland or is known to have met a native Irish speaker, and which for the first time established the Celtic languages as a proper branch of the Indo-European family on rigorous grammatical evidence.
Zeuss's method, reconstructing a Celtic tradition from the actual grammar and vocabulary of its earliest surviving texts rather than from the antiquarian folklore that had substituted for scholarship before him, is the standard the atlas applies to every claim about pre-Christian Celtic religion. Celtic Studies lets the atlas separate what a Celtic myth or ritual can be documented to have actually said in its earliest recorded language from the substantially later romantic and neo-pagan reconstructions built on top of it.