A persian christianity, I'd predict:
c. 100 much of Eastern Persia is converted to Christianity, but much of the West remains Zoroastrian, the religion proving to be very resilent and resisting most efforts of Christianity to convert it...
c. 100-400 Much the same as in our world, except that Persia has degenerated into two states, and the religious fighting between them makes the region as a whole more viatole. The Romans take advantage by permanently conquering Mesopotamia, and the christians there are persecuted and either flee to Persia or a few to Arabia.
Rome is in trouble due to the tensions between traditional polytheisms and Mithraism, and religious wars end in the Mithraic frontier guards from the Danube crushing anti-Mithraic rebellions in Italia and Hispania. German tribes take advantage of this, and invade the empire, though they are driven back at long last by Roman Legions. Britannia, though, is lost forever to British peoples themselves, and they retain the Roman civilization and become an allied state to rome, with a tenuous grip on Pictland and Hibernia.
c. 400 The Epiphthalite Huns crush the civil war ridden Persia and establish the Eastern Hunnic Khanate, while the tensions in Rome erupt as the Huns invade. More massive rebellions scythe through the nation, and the government is hard pressed to keep the invaders at bay. Fortunately, though, the Germans prove useful allies to outflank the Huns, but Greece and Italia are both ravaged before the hordes of Attila are driven back. Rome lies in a weakened state, and loses Dacia, Belgica and the Germanias to the Germans, Mesopotamia to the Eastern Huns, and Armenia to an internal rebellion. The Berbers raid far more frequently in the African territories.
c. 500 Persia manages to throw off the shackles of Hunnic domination, and Christianity is seen as a rallying point to the people. A crusade is launched to Judea, and they conquer it from the weak Roman grasp, and manage to wrest Eastern Anatolia and Egypt from it as well. The only Roman stronghold in Asia now is the Ionian Coast of Asia Minor.
This is about as far as I would go to set up an NES, however, I'll go on for the sake of speculation.
c. 600 Mohammed, influenced by the Christian refugees in his homeland, founds Islam, and tquickly unites the Arab tribes. They attack the Persian Empire en masse, taking southern Mesopotamia, Judea, Egypt, and Anatolia, but are blocked from further expansion by the heavy resistance the Persians put up. Multiple Crusades are launched from Persia, however, they are delayed by renewed attacks of the Gupta Empire (which survived since the Epiphtalite Huns focused their attacks on Persia) and the imperial expansion of Tang China. The last Crusade is launched sometime around 700, and the end result is a stalemate. Both Islam and Persia fracture into small states, while Tang China is finally forced out of their Soighidan stronghold by the Tibetians, and the Gupta are finally broken up by the same.
c. 800 India is now a patchwork of strong states based around the Deccan, the Indus, and the Ganges, The Tibetians have a large and powerful Empire. China is still ruled by the Tang, but they are fearful of rebellion and liable to collapse. Islam is split between Palestine, Arabia, Egypt, and the Berber Caliphate. The only Christian states are Isafahan, Bactria, Soghidana, and Armenia. Rome is reduced to Italia, Southern Gaul, Iberia, and Western Anatolia. Britannia is the last true Roman state, though, and controls all of Roman Britain, even up to the Antonine Wall, but with only a tenuous grip on Pictland and Hibernia, and they must deal with increasing Viking attacks. The Frankish Kingdom consists of Northern Gaul, Belgica, and the Rhine valley, while Germania is another semi civilized state centered on the Elbe. Greece is supposedly Roman, but is in reality ruled by the Goths, whose Empire extends through Thrace, Illyria, Dacia, and even some parts of Scythia. The Vikings have the most civilized nations in all of Northern Europe, including Danemark, Sweden, Norway, Rus, and nominally Hebreidia (though this is often occupied by Britannia).
Most of Europe is Mithraic (except for the Norse parts), Islam rules Egypt, Arabia, and North Africa. Christianity occupies Persia, Mesopotamia, and Armenia, though many people in the area, about 20%, are Zoroastrian, while Bactria is almost entirely Zoroastrian. The east looks roughly as it does in our world, except India is much more Buddhist than Hindu.
) and I dar enot one theological historian will disagree with the fact that if christianity had been in away detianed from the cours eof history it has in our own time line, the males of the wester world would be mostlly mithraic.
crazyeye: ) or southward to South Africa. Or he could try to take on the various peoples of the former Roman empire, especially in North Africa. Islam based in Carthage, anyone? 
