Christianity can be said to have started prior to the birth of the Christ Child, with the Judaic prophesies of the Messiah. It gained momentum with the Ministries of Christ and his thirteen Apostles, and then mushroomed with his supposed rising. Even with this, Christianity was little more than a persecuted cult that spread among the oppressed peoples of the Empire.
Christianity technically starts with the acceptance of Christ as Saviour by the Roman Emperor and his fiat that the Roman Empire shall glorify Christ.
As for the Bible as known today, it covers about 1/3 to just under HALF of the teachings and books for the various Christian sects and cults that populated Asia Minor, Southeastern Europe, Northern Africa. The various leaders met to decide on a common Bible.
While there is much useable data in the Bible that can be used in the study of Ancient Civilisations, the rest of it is essentially fiction and or outright lies.
Remember, the Christian Churches (Catholic or Protestant) generally agree that Christ and his Apostles taught Christian Values through homilies and morality stories...even if some of the stories do have a basis in reality, and stories of the Past commonly known.
For example, Noah's Arc...the story is prevalent through-out the Middle East, and when dug into, leads to the Ruler of a small City State in what is now Iraq. He was washed out to sea with his family and cargo while on a trading mission. Because he failed to deliver his cargo on time, he would have been forced to pay a penalty, that would have put him in debt, and according to the laws of the time, he and his family, Rulers or no, would have been sold into Slavery to work-off the debt...so, they simply set-up shop somewhere along the coast of the Arabian Penninsula. Geological records in the Mountains in Northern Iraq show that there was a massive storm and massive flash floods that would have flooded the Tigris and Euphrates.
As to the number of people involved...due to the length of time since the events in question...no one can, nor likely (without the invention of working Time Travel methods) ever be able to answer.
But, if it's like modern Scientology, two people... L. Ron Hubbard started it to win a bet.
EDIT: Incidentally...I'm a Baptized Anglican and Practicing Agnostic.