Meister Eckhart is an influential, and often surprising, Christian theologian / philosopher / mystic whose writings & sermons have both inspired future thinkers in the catholic church, but also been condemned by the pope as heretical.
Sources / Further Reading:
McGinn, Bernard (2003). The Mystical Thought of Meister Eckhart: The Man from Whom God Hid Nothing. Independent Publishers Group.
McGinn, Bernard (1992). Meister Eckhart: Teacher and Preacher. Classics of Western Spirituality Series. Paulist Press International, U.S.
McGinn, Bernard (1981). Meister Eckhart: The Essential Sermons, Commentaries, Treatises and Defense. Classics of Western Spirituality Series. Paulist Press International, U.S.