Here I am again, stirring up trouble!
This is something that, as a devout Catholic, I've thought about for quite a while. I know that much of the framework for the adoption of Christianity by the Roman Empire came with the reforms of Aurelian in pursuit of unifying the empire under his god Sol Invictus. I know, in fact, that Sassanid Persia seemed to be more tolerant of Christianity in the early years than Rome. That being said, here's my hypothesis:
The Emperor Aurelian (reigned from 270-275 OTL) is not assassinated by rebellious troops. He largely succeeds in uniting the Empire under the henotheistic primacy of his patron deity Sol Invictus. Shahanshah Bahram I of Persia, finds himself under attack from Aurelian's armies. After losing considerable swathes of Asia Minor to the Roman Warrior-Emperor, he falls under a stressful fever. He recieves a dreamlike vision where he sees the flame of Zoroaster extinguished as Roman Troops march up his palace steps. He looks into the sky to see the message "in this sign conquer" etched across an executioner's cross. A arrows rain down from the Cross in the sky, killing the Romans. Bahram comes to in a cold sweat, before proclaiming that Ormazd is a false deity that will lead the Empire to ruin. Their only hope, he proclaims, lies in the God of the Christians. He has crosses (more 'uppercase' T than 'lowercase' +) painted on his men's shields and banners, and sends forth his armies to meet Aurelian. The Romans are crushed, and the Shah has the Zoroastrian clergy executed as hereitcs along with the Manicheans (as per OTL).
Prior to his death in late 276, he calls the Council of Ctesphion with the intent of establishing a United Christian Church. After nearly six months of deliberation, a Canon is selected and the Ctesphion Creed is formed. Rome bans Christianity more harshly, and the Church finds its patronage in Persia. Within five generations nearly all of Persia is Christian.
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So, how plausible does this seem?
PS: Dibs on the TL name (Ad Orientam)