Atheism is commonly understood as rejection of theism in the broadest sense of theism, i.e. the rejection of belief in God or gods. The claim that the existence of any deity is unknown or unknowable is agnosticism.
The term theism derives from the Greek theos or theoi meaning "god" or "gods". The term theism was first used by Ralph Cudworth (1617–1688). In Cudworth's definition, they are "strictly and properly called Theists, who affirm, that a perfectly conscious understanding being, or mind, existing of itself from eternity, was the cause of all other things".
Polytheism is the belief that there is more than one god.
In practice, polytheism is not just the belief that there are multiple gods; it usually includes belief in the existence of a specific pantheon of distinct deities.
Polytheism is also divided according to how the individual deities are regarded:
Henotheism: The viewpoint/belief that there may be more than one deity, but only one of them is worshiped.
Kathenotheism: The viewpoint/belief that there is more than one deity, but only one deity is worshiped at a time or ever, and another may be worthy of worship at another time or place. If they are worshiped one at a time, then each is supreme in turn.
Monolatrism: The belief that there may be more than one deity, but that only one is worthy of being worshiped. Most of the modern monotheistic religions may have begun as monolatric ones, although this is disputed.
Pantheism: The belief that the physical universe is equivalent to god, and that there is no division between a Creator and the substance of its creation.
Panentheism: Like Pantheism, the belief that the physical universe is joined to a god or gods. However, it also believes that the divine pervades and interpenetrates every part of the universe and also extends beyond time and space. Examples include most forms of Vaishnavism and the philosophy of Baruch Spinoza.
The distinction between these two beliefs may be ambiguous and unhelpful, or a significant point of division. Pantheism may be understood a type of Nontheism, where the physical universe takes on some of the roles of a theistic God, and other roles of God viewed as unnecessary.
Classical Deism is the belief that one God exists and created the world, but that the Creator does/do not alter the original plan for the universe, but presides over it in the form of Providence; however, some classical Deists did believe in divine intervention.
Deism typically rejects supernatural events (such as prophecies, miracles, and divine revelations) prominent in organized religion. Instead, Deism holds that religious beliefs must be founded on human reason and observed features of the natural world, and that these sources reveal the existence of a supreme being as creator.
Pandeism: The belief that God preceded the universe and created it, but is now equivalent with it.
Polydeism: The belief that multiple gods exist, but do not intervene in the universe.
Autotheism is the viewpoint that divinity, whether also external or not, is inherently within 'oneself' and that one has the ability to become godlike. This can be in a selfless way, a way following the implications of statements attributed to ethical, philosophical, and religious leaders (such as Mahavira).
Nielsen, Kai (2010). "Atheism". Encyclopædia Britannica. Retrieved 2011-01-26. Atheism, in general, the critique and denial of metaphysical beliefs in God or spiritual beings.... Instead of saying that an atheist is someone who believes that it is false or probably false that there is a God, a more adequate characterization of atheism consists in the more complex claim that to be an atheist is to be someone who rejects belief in God for the following reasons (which reason is stressed depends on how God is being conceived)...
Edwards, Paul (2005) . "Atheism". In Donald M. Borchert (ed.). The Encyclopedia of Philosophy. Vol. 1 (2nd ed.). MacMillan Reference USA (Gale). p. 359. ISBN9780028657806. On our definition, an 'atheist' is a person who rejects belief in God, regardless of whether or not his reason for the rejection is the claim that 'God exists' expresses a false proposition. People frequently adopt an attitude of rejection toward a position for reasons other than that it is a false proposition. It is common among contemporary philosophers, and indeed it was not uncommon in earlier centuries, to reject positions on the ground that they are meaningless. Sometimes, too, a theory is rejected on such grounds as that it is sterile or redundant or capricious, and there are many other considerations which in certain contexts are generally agreed to constitute good grounds for rejecting an assertion.(page 175 in 1967 edition)
Hepburn, Ronald W. (2005) . "Agnosticism". In Donald M. Borchert (ed.). The Encyclopedia of Philosophy. Vol. 1 (2nd ed.). MacMillan Reference USA (Gale). p. 92. ISBN9780028657806. In the most general use of the term, agnosticism is the view that we do not know whether there is a God or not.
(page 56 in 1967 edition)
^Rowe, William L. (1998). "Agnosticism". In Edward Craig (ed.). Routledge Encyclopedia of Philosophy. Taylor & Francis. ISBN978-0-415-07310-3. In the popular sense, an agnostic is someone who neither believes nor disbelieves in God, whereas an atheist disbelieves in God. In the strict sense, however, agnosticism is the view that human reason is incapable of providing sufficient rational grounds to justify either the belief that God exists or the belief that God does not exist. In so far as one holds that our beliefs are rational only if they are sufficiently supported by human reason, the person who accepts the philosophical position of agnosticism will hold that neither the belief that God exists nor the belief that God does not exist is rational.
Halsey, William; Robert H. Blackburn; Sir Frank Francis (1969). Louis Shores (ed.). Collier's Encyclopedia. 22 (20 ed.). Crowell-Collier Educational Corporation. pp. 266–7.
^Cudworth, Ralph (1678). The True Intellectual System of the Universe, Vol. I. New York: Gould & Newman, 1837, p. 267.
^“Monotheism”, in Britannica, 15th ed. (1986), 8:266.
^Webster's New International Dictionary of the English Language (G. & C. Merriam, 1924) defines deism as "belief in the existence of a personal god, with disbelief in Christian teaching, or with a purely rationalistic interpretation of Scripture". Although listed as a type of theism, deism is completely different from theism. If anything, theism would be an off-shoot of deism since it takes beliefs a step further to include miracles and divine revelation, with deism being the "base" belief in God.