The central concept to be addressed in this book is that of ‘ineffability’. The specific way in which I understand the term will, I hope, become clear in the next chapter but, by way of initial orientation, it seems appropriate to make some initial remarks. By the term ‘ineffable’, I mean to refer not just to the concept of that which is inexpressible or in practice unknown but to the notion of what is in principle resistant to conceptual grasp and (therefore) literal linguistic articulation. ‘Ineffability’, in my sense, should therefore be taken also to include a non-disparaging sense of ‘mystery’. It may seem obvious that this concept has long been at work in Christianity, indeed this fact is part of what motivates me to write a book about it. But I first want to distinguish between two lines of religious thought, both of which look, at first glance, like promising sources for the notion of ineffability in Christianity. I label these the ‘theological conception’ of ‘mystery’ and the ‘apophatic premise’. The initial interest in the theological conception is provoked by its attempt to explore the connection between mystery and religious practice (especially ritual), which is one of my eventual aims. Although the mystical writing produced by those committed to the apophatic premise has inspired most of the (very little) existing philosophical treatment of the notion of ineffability in the period of interest, both lines of thought have received twentieth-century and contemporary philosophical defence.