From the earliest centuries there has existed a Christian theology of mysticism, difining the state which Bernard Lonergan called a “being in love with God”. St. John of the Cross wrote such a theology for the sixteenth century, calling it “the science of love”. Now, William Johnston, one of the great spiritual writers of our time, does the same for the twenty-first century in Mystical Theology. Johnston shows how such a theology today must dialogue with modern science and with Eastern religious-and the provides it, engaging Einstein’s theories as well as Zen Buddhism. As he writes it becomes clear how the “science of love” cannot be an esoteric discipline, for monks and nuns alone. In Johnston’s writing it becomes accessible to all modern people grappling with the problems of our age.