In addition to this and other work concerning religious language there was a renewal of fundamental discussion of Christian, and more broadly religious, epistemology. The natural theology tradition held that, in order to be rational, religious belief must be supported by adequate evidences or arguments. It was assumed that God’s existence must be validly inferred from generally acceptable premises. This evidentialist principle was questioned, however, not only by such earlier thinkers as Pascal and William James but also by a number of Christian philosophers of the second half of the 20th century. Evidentialist thinking was foundationalist in granting that there are some beliefs that can be reasonably held directly and not by inference from other evidence-stating beliefs. Thomas Aquinas, for example, recognized self-evident truths and the reports of the senses as basic in the sense that they do not need support from other beliefs. They thus provide the foundations on which a belief structure can properly be built. Belief in the existence of God was not regarded as basic or foundational in this way but was thought to require adequate evidence or arguments. It was argued by Alvin Plantinga that the range of properly basic beliefs is wider than classic foundationalism recognized. It can include not only beliefs about the past and the existence of other persons but also belief in the reality of God. Such beliefs can be basic (i.e., not inferred), and they are properly basic if held in appropriate circumstances. Thus, the belief that “There is a tree before me” is properly basic for one who is having the experience of seeing a tree; and the belief that “God exists” is properly basic for one who experiences God’s judgment, forgiveness, love, claim, providential care, or some other mode of divine presence.

Discussion of this proposal centred upon the criteria for proper basicality: In what circumstances is it appropriate, and in what circumstances not, to hold the basic belief in God or the basic beliefs of other religions or of the naturalistic worldviews?

A related contemporary development, pursued by William Alston and others, is the claim that religious experience constitutes an entirely proper basis for religious beliefs. The claim is not that one can validly infer God as the cause of theistic religious experience, but that one who participates in such experience is entitled to trust it as a ground for belief. It was argued that human beings all normally operate with a “principle of credulity” whereby they take what seems to be so as indeed so, unless they have some positive reason to doubt it. Accordingly, one who has the experience of living in the presence of God can properly proceed in both thought and life on the basis that God is real. Such belief inevitably involves epistemic risk—the risk of error versus the risk of missing the truth. But perhaps the right to believe that was defended by William James applies in this situation.

The discussion focused on the analogies between religious experience and sensory experience in relation to which the principle of credulity is virtually universally accepted. It is uncontroversially proper to hold beliefs reflecting sense experience, but what of beliefs reflecting religious experience? Whereas all human beings hold the former and could not survive without doing so, the latter type of belief seems to be optional. Although beliefs regarding physical objects can be empirically confirmed or denied, religious beliefs cannot. Acknowledging these differences, some Christian philosophers argued that they are to be expected, given the difference between the human relationship to the world and to God. It is necessary to human existence as physical organisms that consciousness of the material environment should be forced upon human beings. On the other hand, it is necessary for existence as relatively autonomous and responsible beings that consciousness of God should not be forced upon them, for to be compulsorily aware of God’s universal presence as limitless goodness and power, making a total claim upon human life, would deprive them of creaturely freedom. Humans are accordingly set at an epistemic distance from God that is overcome only by faith, which can be identified with the voluntary interpretive element within the experience of God’s presence.

The central Christian doctrine of the divinity of Christ was another topic of discussion in the later 20th century. Philosophical questions concerning this topic were debated in the 3rd to 5th centuries, as noted above, in terms of the key notion of ousia/substantia. The concept of substance, however, although confidently used throughout the medieval period, was widely questioned by modern thinkers and found little place in distinctively 20th-century streams of philosophy. Consequently, there was a variety of attempts, in which theology and philosophy mingled inextricably, to find an interpretation that would be intelligible to the modern mind. Instead of the basically static notion of substance—Jesus qua human being of human substance and qua divine of God’s substance—many have preferred the more dynamic idea of divine action. From this point of view Jesus was divine in the sense that God was acting redemptively through him; or, instead of a homo-ousion, identity of substance, between Jesus and the heavenly Father, there was a homo-agapion, an identity of divine loving. Others, however, criticized such alternatives to the older substance language, often on the ground that, whereas “being of the same substance as” is an all-or-nothing concept, divine activity in and through a human life is capable of degrees, so that the divinity of Christ may in principle be de-absolutized.

The problems of religious pluralism were increasingly seen as requiring the attention of Christian philosophers. One reason arises from the kind of apologetic described above, hinging upon the reasonableness of basing beliefs upon religious experience. There is considerable variety within the Christian tradition itself, and in the world as a whole Muslim forms of religious experience give rise to and justify Islamic beliefs, Jewish forms of experience to Jewish beliefs, Hindu to Hindu beliefs, Buddhist to Buddhist beliefs, and so on. These different belief systems include mutually incompatible doctrines. Thus the experiential solution to the problem of justifying Christian beliefs gave rise to a new problem constituted by the conflicting truth-claims of the different religious traditions.

The other reason the great world faiths provided new issues for Christian philosophy was that some of their belief systems challenge long-standing Christian assumptions. Whereas Judaism and Islam raise theological questions, the most challenging philosophical issues are raised by Buddhism. The belief in God as the personal ultimate is challenged by the idea of the ultimacy of the nonpersonal dharma-kaya. The idea of the immortal soul is challenged by the anatta (“no soul”) doctrine, with its claim that the personal mind or soul is not an enduring substance but a succession of fleeting moments of consciousness. And yet Buddhism, teaching as it does doctrines that are radically different from those of the Christian faith, also challenges Christianity by the centrality within it of compassion, peaceableness, and a respect for all life.

These and other issues raised by the fact of religious plurality are ones that Christian philosophers have only begun to face but that suggest the possibility of major developments in Christian thinking.