Well at its root, Christianity means a body that follows Christ. Therefore, there cannot be divisions within a body. Sin causes people to separate, but there is no division in Christ.
Catholic, Orthodox, and Protestant are the three main divisions of Christianity.
Roman Catholicism. .................................... Christianity is the religion. Roman Catholicism is one church or major division within Christianity. The Protestant Church and Orthodox Church are other major divisions.
Any divisions of Christianity were created by men and usually the result of self serving church systems. There should be no divisions in Christianity (since Christ be the head of the church). If we were to discuss two major divisions I would have to point toward Established Church Systems and Protestant Reformation the latter upon which the United States is established. (Read William Bradford)
A movement towards more unity, and less divisions.
I tend to think of Catholicism as a branch of Christianity. In that case, Christianity is the most widely practiced religion worldwide. Catholicism is the largest group / church / denomination / division within Christianity, and I believe it is the largest such group worldwide as well, considering divisions of all other religions.
A subshell is a subdivision of electron shells.
Catholic, Methodist, United Church of Christ, Baptist, and Lutheran
The main divisions are catholic, protestant , charismatic, and evangelical churches.
Within Christianity all people are just considered children of God, thus there are no "social classes" within Christianity.
Subshell or Subshells
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Eastern Europe is mostly Orthodox, which is a branch of Christianity (Greek Orthodox and Russian Orthodox are some examples). However, there are also large Catholic populations throughout.