This dissertation explores the symbiotic relationship that has developed over the last century between two seemingly disparate discourses: Christian mystical theology and phenomenology. My principal concern is the origin and context of this relationship. How and why have we arrived to a situation where atheist philosophers have found helpful resources in the mystical traditions? More specifically, how and why did phenomenologists, beginning with Martin Heidegger, first engage mystical theology in order to formulate post-death-of-God conceptions of the human subject? Finally, how did Heidegger's earliest French readers---the forebearers of deconstruction and the so-called "theological turn" in French phenomenology---accomplish this task in response to his early analyses of the human and its relationship to the world? Answering these questions will help to explain how mysticism became a legitimate philosophical resource for those thinkers who not only do not believe in God, but who are openly antagonistic to such belief, at least in its metaphysical iterations. It will also help to explain how post-Heideggerian deconstructive philosophies became a helpful dialogue partner for those philosophers of religion interested in formulating non-metaphysical theologies by way of engagement with the mystical traditions. In sum, knowing the roots, contexts, and history of the unlikely intimacy of these two intellectual domains will enable thinkers on both sides of the conversation to understand more clearly the stakes involved in theological engagements with post-Heideggerian philosophy and philosophical openness to certain elements of mystical theology.