I believe the question as it's asked in the subject title is could be much better worded. A much better question, IMO, is, "What are the criterion to becoming a Christian?" This not only forces us to study the early Christians and their doctrines, along with Holy Tradition and Scripture, but more importantly it forces us to look at ourselves to see if we meet said criterion.
It may sound judgmental, mean-spirited and against the Gospel message to do so, but to not do so would mean that the Mormon, the Baptist, the Holiness Pentecostal, the Roman Catholic, the Jehova's Witness, the Episcopalian/Anglican and the Eastern Orthodox Christian are merely diverse members of the same body. My friends, only a fool would adopt such an outlook.
There IS an established, well-trodden set of criterion to answer such a question. The quote given from our holy father, St. Theophan the Recluse, holds the key to the answer.