Religion
There is no scholarly consensus over what precisely constitutes a religion. It may be defined as a cultural system of designated behaviors and practices, world views, texts, sanctified places, prophesies, ethics, or organizations, that claims to relate humanity to supernatural, transcendental, or spiritual elements.
Religious Studies
Religious studies, alternately known as the study of religion, is an academic field devoted to research into religious beliefs, behaviors, and institutions. It describes, compares, interprets, and explains religion, emphasizing systematic, historically based, and cross-cultural perspectives.
Spirituality
Traditionally, spirituality refers to a religious process of re-formation which "aims to recover the original shape of man," oriented at "the image of God" as exemplified by the founders and sacred texts of the religions of the world. In modern times the emphasis is on subjective experience of a sacred dimension and the "deepest values and meanings by which people live," often in a context separate from organized religious institutions. Modern systems of spirituality may include a belief in a supernatural (beyond the known and observable) realm, personal growth, a quest for an ultimate or sacred meaning, religious experience, or an encounter with one's own "inner dimension."
Theology
Theology is the critical study of the nature of the divine. It is taught as an academic discipline, typically in universities and seminaries.
Religion
For the majority of English people there are only two religions, Roman Catholic, which is wrong, and the rest, which don't matter.
Duff Cooper, Old Men Forget (1953). London: Rupert Hart-Davis, p. 128.
Spirituality
I, at any rate, acknowledge only one master, not forty-five million two-legged sheep, or two thousand million, but simply and absolutely the spirit.
Olaf Stapledon, Sirius (1944)
Theology
When all is said and done, science is about things and theology is about words. Things behave in the same way everywhere, but words do not. ... Theology works in one culture alone. If you have not grown up in Polkinghorne's culture, where words such as "incarnation" and "trinity" have a profound meaning, you cannot share his vision.
Freeman Dyson, The Scientist As Rebel (2006).