The Evangelical Outpost asks “what constitutes a valid religion?”. Since becoming a born-again Christian about 5 years ago, I have come up with my own definitions of religion. I have two.
The first is the broadest, and simplest to explain. A religion is a world-view. It's how you perceive the world, and how you think it works. Under this definition, everyone on Earth has a religion; be it Christianity, Islam, Judaism, Buddhism, Greek or Roman Mythology, or even Atheism or Secular Humanism.
The second definition is highly influenced by my own Christian faith. This definition says that a religion is how a person reconciles himself to God. He realizes that he has fallen short of God in some way, and tries to rectify the situation. All religions, under this definition, require some form of works to “appease” God. Christianity is the only faith that is not a religion under this definition. Christians realize that, while they are separated from God by their fallen nature, they cannot do anything to reconcile themselves to God. The only reconciliation possible is through the death and resurrection of Jesus Christ, who is God and became Man. We realize that our fallen nature is the consequence of our own sin, that the penalty for sin is death (first physical, then spiritual death in the form of eternal hell), and that God's justice requires that penalty to be paid. Since we cannot pay the penalty, God, in His love for us, became a man to pay that price for us. Christians do no works to reconcile themselves to God, but rather associate themselves with the work that God did for us. This is what sets Christianity apart from all other faiths, and is why I say it doesn't fall under the definition of religion. It's also why I find it offensive* to be called “religious”.
* I base my being offended by somethng on the offender's intent to offend. Most people, when calling me religious, don't intend to offend me, and so I am not truely offended by the charge.