@FMF
Apart from Roman Catholicism and Protestantism, the other well-defined mainstream branches are Anglicanism and Orthodoxy. These four denominations make up a pretty solidly defined core with a clear doctrine (although the Eastern Orthodox Church and the Western Roman Church still don't agree on the filioque).
Armenia was the first nation to officially accept Christianity as the state religion and traces its founding to the second half of the first century, having been founded by two disciples of Jesus. Also, the Thomas Christians trace their founding to an apostle, pre-dating the divisive issue of the filioque. If you are interested in the most likely
historically original forms of Christianity, I suggest you look there, rather than rehashing 1700 years of acrimonious theological disputes among the four main, Catholic, Orthodox, Anglican, and Lutheran, branches.
Then there is a penumbra which includes such groups as Christian Scientists, the Church of Jesus Christ of Latter-day Saints (formerly known as the Mormon Church), and Jehovah's Witnesses, which reject some tenets of the mainstream churches &/or add bits to the main doctrinal tenets.
Given that some denominations deny the divinity of Jesus and still call themselves Christians, whereas others claim that after Jesus died on Calvary he went to America and preached to the 13th tribe, I doubt you will hit upon one single, simple, definition which captures the entire spectrum and to which all people who call themselves Christians would agree.