What exactly is the "true" definition to which you are referring?
This is the best that I have:
"religion" = to be bound again, from "re" - again, and "ligo" - to bind. (from the Greek, I'm told)
"Religion" has always meant "doing" stuff, whether rituals, offerings, or showing submission to some hierarchy.
Christianity is the acceptance of God doing something for us in the person of Himself becoming a man, Jesus Christ, living amongst us, and dying in our place under the curse of our sin so that he is justified in redeemning us for His own forever.
Hence, "Christianity," or "the party of Christ" is not a really reigion at all.