From “History of Europe“ by Gwynedd Dowell
As the Palmyrene and Sassanid Empires fought in the east, in the west the Gallic Empire would thrive while what was left of the Roman Empire would undergo internal turmoil. The Praetorian Guard would be purged by the Emperor Marius the Geat shortly after having been installed by them in 580 AD of the Christian calendar, correctly believing that they had grown too powerful and would remove him the moment it was useful to them. However, by this point, a new power had emerged in Rome; the Visigothic tribes that now ruled significant parts of the Empire on behalf of the Emperor, including all of North Africa. Marius would placate them by making the King of the Visigoths Magister Militum[1]. Marius’ son and successor, Quintus Marianus, would prove to be far weaker than his father, and would designate more and more powers to the Magister Militum, that being King Amalric of the Visigoths. Amalric and his successors would become significantly Romanized, endearing them more to the Roman public, and would effectively turn the Roman Emperors into puppets.
The period of Gothic rule would end with the conquest of North Africa by the successors of Abu Yusuf, when Magister Militum Thorismund was deposed by the Roman Flavio Ursino[2] Meanwhile, to the west, the Hispanian Republic founded by Cnaeus the Mad had gradually stabilized into what was effectively a military dictatorship. When the Arab armies invaded Hispania, they brought with them their religion of Christianity. Thus the Iberian peninsula would thenceforth be divided in two, a small Hellene nation in the northwest called Hispania, and a larger Christian nation in the rest of the peninsula called Betica[3].
From “The Göktürks: The History of a People”
The fall of the Sassanid Empire was ironically the best thing that happened to Apostolic Christianity among the Turks. Before, Apostolics were viewed as agents of the Turks’ Persian enemy, but now they were seen as just another religion. Apostolic Christianity would come to replace the worship of Tengri as the predominant religion among the Göktürks. Aside from Apostolic Christianity, Mahayana Buddhism also established a noticeable plurality among the Turks, and Buddhist influences can be found in the Turkestani Church even today. When a culturally Turko-Persian army would invade and conquer large swathes of India centuries later, Buddhist and Hindu beliefs were reimagined in a Christian context[4]. The Turks would nevertheless become enthusiastic patrons of the Apostolic Church, spreading the religion northward and westward to Russia. The Turkic Khaganate comflicted with the Arab Empire just as much as they did with the Sassanids before them, and their newfound devotion to Apostolic Church gave them a justification to liberate their fellow Apostolics in Persia from the Ebionite yoke. With the increasing instability of the Arab Empire, it was now only a matter of time.
[1]”Master of soldiers”, or the top-level military command of the late Roman Empire
[2]Flavius Ursinus. By this point, the Latin language is effectively extinct, replaced with a Romance language, but still used in some formal settings.
[3]Derived from “Baetica.”
[4]Think of the coexistance between Islam and Hinduism in India IOTL