Din (Arabic) is defined a life-way or religion particularly in Islam. The word as used in the Qur’an is probably derived from a Christian source which had previously been barrowed from the Iranian den, religion. Other Arabic meanings are judgment or retribution (as in yaum al-din, day of judgment); custom or usage (A. Jeffery, Foreign Vocabulary of the Qur’an (1938) 131-3). The idea of debt or obligation came to signify judgment on the one hand, and on the other, the duties to be fulfilled–“religion,” such as faith and practice (Qur’an 39. 14).
In what is generally thought to be the final verse on the Qur’an in chronological order Allah is represented as saying, “This day have I perfected for you your religion (dinukum) and chosen for you Islam as (your) religion (Qur’an 5. 4). Thus “The religion before Allah is Islam (Qur’an 3. 19), yet there is no compulsion in religion” (Qur’an 2. 256). Islam is called din al-haqq, the religion of truth (Qur’an 61. 9; 28. 28). The entire system is sometimes referred to din wa-dawla, religion and state (combined), there being no distinction within Islam between religion and politics. A.G.H.
Bowker, John, The Oxford Dictionary of World Religions, New York, Oxford University Press, 1997, p. 284