Theism (Gr. theos, 'God'), etymologically equivalent to belief in a god or gods, and as such opposed to Atheism (q.v.), is now usually understood to mean the doctrine of the One, supreme, personal God, 'in whom we live, and move, and have our being'—as distinguished from Polytheism (q.v.), which recognises more gods than one; from Pantheism (q.v.), which denies the divine personality; from Agnosticism (q.v.), which denies that we can know anything of God; and from Deism (q.v.), which, etymologically equivalent to Theism, is generally defined as recognising the personality of God, but denying His providence and active presence in the life of the world (though it should be remembered that many deists would not have admitted that this was their doctrine). Deism further explicitly rejects revelation and trinitarian conceptions of the godhead, while Theism may or may not accept these doctrines. But the term theism was often used as equivalent to deism with its negative postulates: and when Theodore Parker speaks of 'Theism,' or when the Brahmo-Somaj is called the 'Theistic Church of India,' or a book is published with such a title as Theistic Devotions, that kind of theism is also meant which either denies or at least does not include Trinity, incarnation, inspired and infallible revelation, or miracle.
Various views of the origin of the notion of God, and the relation of the monotheistic religions to nature-worship, animism, spiritism, and polytheism, have been indicated in the article Religion (q.v.); together with several definitions of religion as man's attitude towards God.
Theism as the doctrine of the nature and attributes of God covers a large part of the field of theology and speculative philosophy. But in practice it is usually restricted to the maintenance of the thesis that God may be known; the history of the origin and development of the idea of God; and the statement, criticism, and defence of the arguments for the existence of God. The main part of its work is apologetic, in opposition to the hostile systems and theories, rather than a scheme of systematic Theology (q.v.). No competent apologist now stakes the existence of God on any one argument, or exhibits the proof as a series of syllogisms. It is rather maintained that the study of human history, of human nature especially on its moral and spiritual side, and of the world as far as science reveals it to us make for the existence of a God, demand such a postulate as the key to the universe, and render the belief in a personal God greatly more probable than any other thesis—a subject vastly too wide for discussion here. But it is necessary to name what are often referred to as the four great arguments for the existence of God. (1) The ontological argument first formulated by St Anselm proceeds from the notion of a most perfect being to infer his existence; without actual existence the idea would fall short of perfection. The argument was re-stated in a different shape by Descartes (q.v.) and by Samuel Clarke, and, though very contemptuously treated by Kant, is still an element of the argument that without a God the world is a chaos.
(2) The cosmological argument, employed by Aristotle, Aquinas, and a host of Christian authors, is an application of the principle of Causality (q.v.). We cannot conceive an infinite regression of finite causes; therefore beyond the last or first of the finite causes is the Infinite. From motion the argument is to a mover.
(3) The teleological argument, or argument from design, proceeds from the order and arrangement of the universe, the reign of law and beauty and adaptation, to the intelligent and supreme fountain of order. This is the most familiar of the arguments, especially on the lines laid down by Paley.
(4) The moral argument was that relied on by Kant (q.v.) when he destructively criticised the other three, and forms a part of most modern theistic arguments. God is a postulate of our moral nature; and the moral law in us implies a lawgiver without us.
See, besides general works on apologetics and dogmatic theology, Professor Flint's Theism (1877; 4th ed. revised, 1889); Harris, The Philosophical Basis of Theism (New York, 1883); The Grounds of Theistic and Christian Belief (New York, 1883); the Duke of Argyll, The Reign of Law (1866; 19th ed. 1890); Kant's Critique of Pure Reason; Mill's Three Essays; Janet's Final Causes (trans. 1878); and the Gifford Lectures (1888 et seq.).