Is that it's not aptly named.
A good portion - and I'd say most - of the religion are practices, rituals, rules and philosophies that were never spoken by Christ, or endorsed by Christ.
I think there are some sects that are strictly about the teachings of Christ, who I feel was an ascended being and someone whose words matter, but they're not mainstream Christianity.
Define 'mainstream' Christianity.
How do you know what all of Christ's teachings are and ever will be? You are not Jesus Christ or God. I really doubt you know every parable ever used, or ever will be used by either of them. There really is only a few running themes in all that has been taught from them:
* The kingdom of God is everlasting. It will never cease nor be destroyed.
* A common theme spoken by both of them is about freedom of choice, given to all Men. Note that is not freedom from the consequences of that choice.
* Satan, who counsels no choice, is an enemy of the God and of Man. He will be cast out, forever. His council will fall, and so will his influence.
* Men and women are children of God, imperfect though we are, because of the Fall, and because of death that was brought into the world with it.
* Jesus Christ has the capability to pay for the effects of the Fall, allowing men to achieve perfection. Death is not permanent. Men may be resurrected and enjoy everlasting life and even become perfect once again, just as Christ Himself was. This indeed is the glorious message that Easter celebrates.
A Christian is one that tries to follow Christ's teachings, and God's teachings with it.
Merely a belief that Christ exists doesn't cut it. Satan, after all, believes that Christ exists and that he is the Son of God, even tempting Christ and fighting against everything Christ stands for.
Do not presume to usurp the judgment of God.