Hey! I talked about pre-Islamic Arabian polytheism and monotheism in a reply to What was(were) the religion(s) before Islam in North Africa and Middle East ? and /u/shlin28 gave a great response on Christianity in this same time period. Hopefully that gives you a nice start!

Two strains of Christianity which were much more common in that region than the rest of the world were the Church of the East and Oriental Orthodoxy.

As u/frogbrooks states, u/shlin28 gives a very good explanation of them. I'll just summarize the distinctive theological positions of those two branches of Christianity.

After the Trinity was determined to be orthodoxy, the next question was whether or not Christ had a human nature in addition to a divine one and what the relationship between the two is. Although one position, the Monophysite position, asserted Christ to have a single divine nature and no human nature, the first major schism relevant here was when a guy named Nestorius taught that Christ had two separate natures, one human and one divine (interestingly, Nestorianism rejected the title theotokos, meaning "mother of God" or more literally "God-bearer" for Mary because she was only the mother of the human portion). Most of the Christian world at the time rejected this as heresy, but the Church of the East, which mostly covered the land outside of the Roman Empire's eastern border, accepted it and its remnant still considers Nestorius to be a saint. After this, the Council of Chalcedon occurred, in which the main resolution was to define Christ as having two natures joined in hypostatic union. The predominately Greek and Latin churches that adopted this christology would have a later schism into being the Catholic and Eastern Orthodox Churches. The churches that rejected this were mostly in what we would now consider to be the Middle East, particularly the Syriac and Coptic churches. Their christology defines Christ to be of one nature that is both human and divine. The term for this is Miaphysite.