I have looked at many definitions of religion and this one by Thomas Nagel works quite well.
‘To better identify the question, we should start with the religious response. There are many religions, and they are very different, but what I have in mind is common to the great monotheisms, perhaps to some polytheistic religions, and even to pantheistic religions that don’t have a god in the usual sense. It is the idea that there is some kind of all-encompassing mind or spiritual principle in addition to the minds of individual human beings and other creatures—and that this mind or spirit is the foundation of the existence of the universe, of the natural order, of value, and of our existence, nature, and purpose. The aspect of religious belief I am talking about is belief in such a conception of the universe, and the incorporation of that belief into one’s conception of oneself and one’s life.’ [Nagel – Secular Philosophy and the Religious Temperament; pp.44-5]