Religion is wherever one's Ultimate Value is found, no matter what that Ultimate Value is. Every moment of our lives, our every action and non-action chooses one value over another. Whichever value is chosen over all others is our religion.
Religion is not just whatever resembles Christianity. Religion can be radically different from Christianity, even though many Pagans never experienced radically different religions to realize it.
Some definitions of religion from those who study it:
Religion as religion has to do with ultimacy. And there is room for religion to be radically different from Christianity and still be religion.
Some folks have thought of Paganism as a "spirituality" rather than religion, while others have thought of it as a "lifestyle" rather than religion. It can be--for them. And religion can be religion for some and spirituality or lifestyle for others, it all depends on whether the individual treats it as ultimate or as less than ultimate. If it is less than ultimate, then the question becomes, "Then what IS the ultimate for this person?"
Most religions are ultimate because they bring together into one a number of important pieces, ideas about what is ultimate reality, a way of life to live in light of ultimate reality, an ethic by which to live with others and with the rest of the world, ritual activities to reinforce the way of living, a "spiritual" way of feeling that changes or reinforces the relation with ultimacy. For some people one of these piece parts, such as ethics or spirituality can be emphasized, but others can place rather equal emphasis on all the parts.
"When I mention religion, I mean the Christian religion; and not only the Christian religion, but the Protestant religion; and not only the Protestant religion but the Church of England."
Henry Fielding, Tom Jones, book 3, chapter 3