What if on the border the legions on the border with Germania pass on Mithraism to some of th elocal German Legionnaires, maybe Herman the German decides to adopt it, and then (after his great victory in the Tuetonwald), the religion is widely adopted as a warriors religion.
Later, sometime around 150 AD, a female component is added to the religion in the East, maybe based on a composite of Isus and Frejya. A Yin-Yang sort of dicotomy is adopted in the religion, and by 320 A.D most of the Germanic tribes have adopted the religion.
When the Huns move West, the now better disciplined Goths are able to withstand the onslaught, but determine to move West to find more peaceful lands. They approach the Eastern Empire, and while some serve in the Eastern Legions, most determine that the now less tolerant Christianity adopted by the Eastern conflicts with there Mithraism. After a result they move on, but only after they cause a great deal of problems throughout the Adriadic area.
The Eatern Germans, including the Goths, Vandals and related tribes move up along the border of the Empire, and finally enter into North Italy and What is now Switzerland.
The now resurgant Huns begin to threaten the Western Christian Empire, but the Goths, seeing no profit in defending Christians, particularly those who are bankrupt, move on, except for a large number who remain in North Italy (as in OTL.)
However, by now the brand of Mithrism adopted through by ther German tribes includes mysteries that can only be understood in the language of the Goths or other Germans. Since the male aspects of the religion are warrior related, the Goths begin to adopt Latinate locals into their ranks only after the adopt Mithraism and understand the mysteries.
AS in OTL, the Vandals and then the Goths enter into Spain and Southern France, where within a decade or two, local Christians begin to convert into the faith. Since the rreligion is closely assocoiated with the language, Spanish is never adopted as the language of the rulers, and except for some remnant terms used in farming and husbandry, is soon extinct except for some mountainous areas.
Meanwhile, in the East Mohamat has begun to preach, and son a desert whirlwind of steel is launched throughout Egypt, Arabia, and Palestine soon fall to the forces of Islam, but they crash against the forces of the Eastern Empire, and temporarily stop, then redirect themselves against the Vandals in North Africa.