The notion of ineffability (of that which is, in principle, resistant to conceptual formulation and therefore literal linguistic articulation) has been largely ignored by philosophers. The notion is clearly a central one in the Christian mystical tradition and in more recent apophatic theological developments. Mid-twentieth-century philosophical discussions of mysticism invoked the idea and a number of phenomenologists share a sense that the meaningful human world is answerable to some 'background' that is inarticulable and mysterious. But, despite this, the logical implications of the notion of ineffability for religious experience, language and practice have not been explicitly and systematically thought through. This thesis, restricted to a dual focus on twentieth-century and contemporary philosophy and on the Christian religion, attempts to address this neglect. After reviewing the philosophical, and some theological, literature on the notion of ineffability in religion, this thesis identifies a philosophical tension between the notions of 'ineffability' and 'answerability', between the idea that the ineffable is beyond conceptualization and that some kind of experience, language or practice connected with it is required if the notion is to be meaningfully invoked, let alone (as in a religious context) serve as the measure for the meaning of the human world. In this connexion, the meaning of the word 'God' is interpreted as a reference to the concept of ineffability. A recent philosophical defence of the concept is endorsed which, rooted in existential phenomenology and Heidegger's later philosophy, resolves this tension. The detail of theological attempts, by Paul Tillich and John Macquarrie, to accommodate this line of thought as directly inherited from the phenomenologists (especially Heidegger) is examined and criticized, as, eventually, is a more promising theological possibility represented by the neglected philosophy of Karl Jaspers. The rational status of existential phenomenology (its relation to discourse conditioned by the subject-object dichotomy) is examined more closely in the light of the latter criticism. It is concluded that phenomenology's specifically philosophical way of evoking the ineffable is necessarily that of a rationally-based dialectic. The final chapter points to spheres outside philosophy, aesthetic and ritual, which cultivate the experience of the ineffable without such dialectic. It offers an 'aesthetic account of ritual meaning' and concludes by showing how the Eucharistic rite can be philosophically understood as a vehicle for religious experience and expression - the evocation and invocation of the ineffable God.