Theism, in its most inclusive usage, is the belief in at least one deity. Some narrower usages specify that the deity believed in be a distinct identifiable entity, thereby being contrasted with pantheism. Other narrower usages specify that the deity (or deities) be an active, immanent force in the universe, thus excluding some forms of deism. Theism can be categorized into more particular types, such as monotheism and polytheism.
‘Atheism’ means the negation of theism, the denial of the existence of God. I shall here assume that the God in question is that of a sophisticated monotheism. The tribal gods of the early inhabitants of Palestine are of little or no philosophical interest. They were essentially finite beings, and the god of one tribe or collection of tribes was regarded as good in that it enabled victory in war against tribes with less powerful gods. Similarly the Greek and Roman gods were more like mythical heroes and heroines than like the omnipotent, omniscient and good God postulated in medieval and modern philosophy.
‘Agnostic’ is more contextual than is ‘atheist’, as it can be used in a non-theological way, as when a cosmologist might say that she is agnostic about string theory, neither believing nor disbelieving it. Agnosticism is the philosophical view that the truth value of certain claims – particularly metaphysical claims regarding theology, afterlife or the existence of deities, ghosts, or even ultimate reality – is unknown or, depending on the form of agnosticism, inherently impossible to prove or disprove.