Although Neoplatonism was the major philosophical influence on Christian thought in its early period and has never ceased to be an important element within it, Aristotelianism also shaped Christian teachings. At first known for his works on logic, Aristotle gained fuller appreciation in the 12th and 13th centuries when his works on physics, metaphysics, and ethics became available in Latin, translated either from the Greek or from Arabic sources. Aristotle’s thought had a profound impact on generations of medieval scholars and was crucial for the greatest of the medieval Christian thinkers, St. Thomas Aquinas (c. 1225–74). One of Aristotle’s ideas that particularly influenced Thomas was that knowledge is not innate but is gained from the reports of the senses and from logical inference from self-evident truths. (Thomas, however, in distinction from Aristotle, added divinely revealed propositions to self-evident truths in forming his basis for inference.) Thomas also adopted Aristotle’s conception of metaphysics as the science of being. His doctrine of analogy, according to which statements about God are true analogically rather than univocally, was likewise inspired by Aristotle, as were his distinctions between act and potency, essence and existence, substance and accidents, and the active and passive intellect and his view of the soul as the “form” of the body.

Thomas Aquinas’s system, however, was by no means simply Aristotle Christianized. He did not hesitate to differ from “the Philosopher,” as he called him, when the Christian tradition required this; for whereas Aristotle had been concerned to understand how the world functions, Thomas was also concerned, more fundamentally, to explain why it exists.

With the gradual breakdown of the medieval worldview, the nature of the philosophical enterprise began to change. The French thinker René Descartes (1596–1650) is generally regarded as the father of modern philosophy, and in the new movements of thought that began with him philosophy became less a matter of building and defending comprehensive metaphysical systems, or imagined pictures of the universe, and more a critical probing of presuppositions, categories of thought, and modes of reasoning, as well as an inquiry into what it is to know, how knowledge and belief are arrived at in different areas of life, how well various kinds of beliefs are grounded, and how thought is related to language.

There has long ceased to be a generally accepted philosophical framework, comparable with Neoplatonism, in terms of which Christianity can appropriately be expressed and defended. There is instead a plurality of philosophical perspectives and methods—analytic, phenomenological, idealist, pragmatist, and existentialist. Thus modern Christianity, having inherited a body of doctrines developed in the framework of ancient worldviews that are now virtually defunct, lacks any philosophy of comparable status in terms of which to rethink its beliefs.

In this situation some theologians turned to existentialism, which is not so much a philosophical system as a hard-to-define point of view and style of thinking. Indeed, the earlier existentialists, such as the Danish philosopher Søren Kierkegaard (1813–55), vehemently rejected the idea of a metaphysical system—in particular, for 19th-century existentialists, the Hegelian system—though some later ones, such as the German philosopher Martin Heidegger (1889–1976), developed their own systems. Existentialists are identified by the appearance in their writings of one or more of a number of loosely related themes. These include the significance of the concrete individual in contrast to abstractions and general principles; a stress upon human freedom and choice and the centrality of decision, and hence a view of religion as ultimate commitment; a preference for paradox rather than rational explanation; and the highlighting of certain special modes of experience that cut across ordinary consciousness, particularly a generalized anxiety or dread and the haunting awareness of mortality. Existentialists have been both atheists (e.g., Friedrich Nietzsche and Jean-Paul Sartre) and Christians (e.g., Kierkegaard, the Protestant Rudolf Bultmann, and the Roman Catholic Gabriel Marcel). It would be difficult to identify any doctrines that are common to all these thinkers. Existentialist themes have also been incorporated into systematic Christian theologies (e.g., by John Macquarrie).