The attempt to understand the concepts involved in religious belief: existence, necessity, fate, creation, sin, justice, mercy, redemption, God. Until the 20th century the history of western philosophy is closely intertwined with attempts to make sense of aspects of pagan, Jewish, or Christian religion, whilst in other traditions such as Hinduism, Buddhism, or Taoism, there is even less distinction between religious and philosophical enquiry. The classic problem of conceiving of an appropriate object of religious belief is that of understanding whether any term can be predicated of it: does it make sense to talk of it creating things, willing events, knowing things, or being good, or caring, or being one thing or many? The via negativa or negative theology claims that God can only be known by denying ordinary terms any application to it (or them); another influential suggestion is that ordinary terms only apply metaphorically, and that there is no hope of cashing the metaphors. Once a description of a Supreme Being is hit upon, there remains the problem of providing any reason for supposing that anything answering to the description exists. The medieval period was the high-water mark for purported proofs of the existence of God, such as the Five Ways of Aquinas, or the ontological argument of Anselm. Such proofs have fallen out of general favour since the 18th century, although they still sway many people and some philosophers.
Generally speaking, even religious philosophers (or perhaps they especially) have been wary of popular manifestations of religion. Kant, himself a friend of religious faith, nevertheless distinguishes various perversions: theosophy (using transcendental conceptions that confuse reason), demonology (indulging an anthropomorphic mode of representing the Supreme Being), theurgy (a fanatical delusion that a feeling can be communicated to us from such a being, or that we can exert an influence on it), and idolatry, or a superstitious delusion that one can make oneself acceptable to the Supreme Being by other means than that of having the moral law at heart (Critique of Judgement, ii. 28). The warm touchy-feely tendencies held in contempt by Kant have, however, been increasingly important in modern theology (see, for example, buber, tillich).
Since Feuerbach there has been a growing tendency for philosophy of religion either to concentrate upon the social and anthropological dimensions of religious belief (see also magic, language game), or to treat it as a manifestation of various explicable psychological urges. Another reaction is to retreat into a celebration of purely subjective existential commitment (see also Kierkegaard). But the ontological argument continues to attract attention, and modern anti-foundationalist trends in epistemology are not entirely hostile to cognitive claims based on religious experience.