What Is The Largest Christan Religion?

Hi.  I am confused by your question.  When we use the word Christian and Religion in the same sentence, that seems incorrect.  You are either a Christian or you are not.  Religion, according to dictionary.com is;
"a set of beliefs concerning the cause, nature, and purpose of the universe, esp. When considered as the creation of a superhuman agency or agencies, usually involving devotional and ritual observances, and often containing a moral code governing the conduct of human affairs."
Christianity is being a follower of Jesus Christ.  
I hope this does not sound argumentative. Let me know.