There can be no precise and agreed definition of religion. The origin of the word is of little help, for it descends from the Latin religiare, to bind, which suggests the broadest possible boundaries for the territory of religious belief and encourages the acceptance of the argument, frequently put in the twentieth century, that many kinds of belief which fall outside the bounds of the recognized religions, including forms of Marxism and nationalism, have the essential characteristics of religion. It is thus genuinely difficult to define religion for the purpose, say, of teaching children about comparative religion or of formulating laws against offending people's religious beliefs. Are witchcraft and paganism a religion or set of religions? Is theosophy?
However, if the boundaries of religious belief are difficult to draw, the core territory is relatively easy to characterize. Religion is concerned with the worship of transcendent or supernatural beings whose existence is outside or above the realm of the normal, which is mortal and temporal. In its most historically important and ethically demanding form, monotheism, as exemplified in the Jewish, Christian, and Islamic religions, the religious concern is concentrated onto a single God who is omnipotent, omnipresent, and omniscient, the creator of the universe.
Religion is therefore normally of huge ethical significance. What people ought to do is derivable from the existence, nature, and will of God. It would be difficult to be seriously religious in any sense without that religion determining some of one's political beliefs. Indeed, the most natural relation between religion and politics is one in which the most important political questions have religious answers: the legitimacy or otherwise of regimes, the limits of a particular authority, and the rightness or wrongness of legislation can all be derived from religious revelation (see e.g. medieval political theory). The range of religiously justified regimes can be divided into theocracies, where divine revelation and the priests who interpret it rule directly, and those non‐theocracies where the divine will has, nevertheless, sanctioned the particular form of secular rule (the doctrine of the divine right of kings to rule being a typical form of religious, though non‐theocratic, legitimation).
However, since the seventeenth century Western Europe and the Americas have been dominated by secular views which sought successfully to separate religion from politics, so that the state's existence is not justified by theology. Secularization arose out of the tension between science and religion and the schisms between forms of Christianity. It was essential to put religion beyond the sphere of truth and refutation and to justify the authority of the state without recourse to (disputed) theological premisses. Thus in ‘Christendom’, though not in the territory of Islam, there developed an acceptance that political disputes must be resolved on secular grounds. Paradoxically, this process evolved most rapidly in England, which retained (and continues to retain) an established Church (see Church and State).
In a ‘secular’ society the principle that religion and politics are independent realms is accepted, but religion continues to influence politics in a number of ways. Although religious doctrines may be taken to be arbitrary or indeterminate on many political questions, there remain issues on which a Church must speak clearly and forcefully. Roman Catholic doctrine on abortion is one of the clearest cases. A particular form of religious belief can be strongly linked to national identity, as Catholicism has been for the Poles and the Irish, and Orthodox Christianity for the Armenians and Georgians. Where parties are freely formed, there are likely to be parties based generally on Christian social morality, like the many Christian Democratic parties of contemporary Europe, or specifically on one Christian Church. For example, the Catholic People's Party in the Netherlands and the Mouvement Républicain Populaire in France have been specifically Roman Catholic parties, though both of these parties have now merged with others and there has been, since the late twentieth century, a tendency for parties based on one Christian Church to decline.