Arend’s Gifford Lectures, ‘Critique of Heaven and Earth’, delivered in Aberdeen in 1970 and 1972, were published in two volumes: Critique of Heaven (1972) and Critique of Earth (1974). The series sets out to address something of a revolution in the domain of natural theology through an investigation of the works of Karl Marx. Arend identifies the focal point of his enquiry in Marx’s transformation of the critique of religion into the critique of right, the critique of theology into the critique of politics or, more broadly, the critique of heaven into the critique of earth. Paying close attention to Marx’s transformations, Arend examines the origins and development of Marx’s thought through both his (Marx’s) earlier and more mature writings as a basis for his own indication, in light of the Marx-based analysis, that traditionally conceived natural theology be transformed into the critical theology he has in mind throughout his series of lectures. Arend justifies his approach in recognition of an imperative need for such an enterprise in the theological climate at the time of his lectures.