In The Evolution of Religion, vol. 1, Caird establishes the continuum of the science and evolution of religion and endeavours to show that the definition or idea of religion cannot be found in any element common to all religions, of which the author discusses both objective and subjective types. In addition to summarizing views of subjective and objective religion, Caird discusses the moral strength of subjective religion and the opposition of the spirit to nature. He also addresses the transition from a national to a universal religion as well as the rise of moral individualism. Finally, Caird explores the idea of the covenant between God and humankind, the limits and development of Jewish religion, and Jewish history as an illustration of the principle of development.

In The Evolution of Religion, vol. 2, Caird establishes Judaism as a religion of subjectivity. From Judaism he moves to Christianity, the teaching and the work of Christ, who fulfilled and abolished the religion of Israel. Having summarized the teachings of Jesus, the author considers the way in which St. Paul at once generalized and idealized the faith of Christ, liberating it from the Judaic conditions of its origin and at the same time lifting it into the region of theology. Caird further discusses the general characteristics of the evolution of Christianity in post-apostolic times and the development of Christianity before and after the Reformation. Finally, he notes the hindrances and aids to faith in the present day, 1891–1892.