I've spent a few hours contemplating alternatives to Christianity amongst the Germanic tribes. The mystery religion that stood out the most was the Cult of Isis/Isiacism, which was semi-popular across the Mediterranean Basin, because at least two kings of the Alemanni (Mederic and Agenaric/Serapio) practiced it in the mid-4th century.
The defeat and death of Agenaric-Serapio at the Battle of Strasbourg in 357 seems to have discredited Isis/Serapis-worship amongst the Alemanni. The logical 'fix' to this, then, would be to have the Alemanni defeat the Romans and overrun Gaul, legitimizing Isis/Serapis as the 'religion of victory'. The problem with this is the mass enslavement and resettlement of Romans that the Alemanni were performing at the time; thus, a massive victory would only result in the resettlement of Romans in Alemanni territory; as Christianity was stronger than Isiacism, I'm pretty sure this would only strengthen Christianity among the Alemanni and undermine any gains the Isis Cult makes.
Thoughts on a solution?
@Skallagrim, since you seem to be fairly well informed about the period.
I have serious doubts about the prospects for a Cult of Isis among the Germanic peoples-- at least with this POD (although it would require a lot of wrangling, or 'steering the time-line', in any case). We should note that though he was (we may be pretty sure) the brother of Chnodomar (who appears to have been the high king of the Allamanic confederation), Mederic was a so-called 'petty king' (we should really just say chieftain) of one of the Allamannic tribe-bands. He was introduced to Greek philosophy and cultism in Gaul, and he was really into it. Whether the rest of his kinsmen shared his enthousiasm is... pretty doubtful.
His son Agenaric (a.k.a. Serapio)
was into it, obviously. And he was second-in-command to his uncle Chnodomar in military affairs, so we may argue that he was in a sufficiently influential position to extoll the virtues of his faith. Problem: it was very much just that-- a faith. Not a religion. It lacked the organisational structure that would be needed to really "take over". (Which is why I doubt the notion that Mederic and Agenaric represented some kind of whole-sale conversion of the tribe. I think the Hellenic cultism was preference that was pretty much unique to them specifically.)
Even if Chnodomar and Agenaric win a stunning victory at Straßburg and even if this prompts Chnodomar to join the Cult of Isis on the spot and even if the Allamanic confederation ultimately decides that this is the religion for them because it brings victory and glory and therefore must be true...
...They're still just one people, surrounded by neighbours who don't share their beliefs. There is no real impulse to set up a Church (because the old Germanic faith didn't have one, either), so I see the cult becoming (at most) the ethnic faith of the Allamans in particular. Which will isolate them, and most probably lead to their defeat and forced conversion in the longer term.
Ultimately, I feel that the only practical way to out-compete Christianity is to play at the same level. Christianity had several advantages. Number one, it had an organised Church hierarchy, which is great. Number two, it had official literature that was compact, explicitly holy, and very-well suited to preaching the doctrine to both the faithful and the prospective converts. Number three, it openly embraced the poor and downtrodden, and that means you automatically have
numbers on your side. Number four, it had that youthful zeal, prompting both an aura of doctrinal purity and real commitment amongst the faithful. (People who are willing to be eaten by lions for their faith tend to leave more of an impression than people who are willing to throw others to the lions-- funny how that works.) Number five, it was actually
demanding of the faithful, which is what people actually
want in a religion. (It has to
challenge the soul. Otherwise you might just as well join a social club.)
I'm sure there's more to consider, but this paints the picture.
Julian actually understood this fairly well. He not only did his utmost to create a fusion of Greek philosophy, Neo-Platonist mysticism and traditional Greco-Roman faith and mythology-- he actively structured it to be able to compete with Christianity. He wanted the priesthood organised and structured in a hierarchical organisation, as with Christianity. He re-imagined the temples to be more like Christian churches in function, with services and rituals that were more like the ones that yielded such success for Christianity. He wanted to compile a 'canonical' version of the Greco-Roman mythology, infused with philosophical ideas, so that it could be a counterpart to the Christian scripture. He specifically created legislation that would make caring for the poor the exclusive domain of
his priesthood (trying to push the Christians "out of the alms business", so to speak).
So
@stevep has a point there: Julian was probably the best bet. He wasn't some fellow trying to "revive the old ways". What he was trying to create was something dramatically different, and probably capable of succeeding, had he lived (and had he been able to get enough key figures to join his cause, which isn't unrealistic).
This would mean that by default, Julian's religion probably makes it big among the Germanic peoples, too, eventually. Or just as plausibly, they end up adapting their own existing mythology and merge it with the same kind of philosophical notions, forging a "Germanic sister religion" to the one Julian created. Yggdrasil as the cosmic axis mundi, the Germanic deities like
emanations of a greater encompassing divinity, the nine realms of Germanic myth re-cast as layers of reality, closer to and further from the true Forms that lie beyond...
It's not that hard to imagine.
