https://en.wikipedia.org/wiki/Odin#Roman_era_to_Migration_Pe... says "The earliest clear reference to Odin by name is found on a C-bracteate discovered in Denmark in 2020."

So, "written mention of Odin" seems to mean written as "Odin', and not as "Mercury".

It also mentions some debate over if the Goths actually worshiped Odin/Mercury, but I am too ill-informed to make sense if that's relevant.

I did manage to find a scholarly reference to the topic at https://helda.helsinki.fi/server/api/core/bitstreams/c11e69e... ("Pre-print papers of THE 18TH INTERNATIONAL SAGA CONFERENCE SAGAS AND THE CIRCUM-BALTIC ARENA. Helsinki and Tallinn, 7th–14th August 2022")

> The problem of Mercury

> Despite lack of Germanic evidence for the existence of a cult of Wodan/Óðinn before the fifth (or perhaps even the sixth) century, as presented above, many scholars maintain ancient roots. For example, disregarding critical scholarship on various individual sources, Schjødt reiterated that “taken together, they strongly indicate that Óðinn, although not exactly the same as the god that we know from the Nordic sources, has roots reaching far back in time, probably as early as the Indo-European era (at least 3000 BC)” (Schjødt 2019: 67). ...

> The reading of interpretation romana maintains that Tacitus’ famous description of Germanic deities, Deorum maxime Mercurium colunt, should be understood to signify Wodan/Óðinn. Of course, it has already been shown by Karl Helm that this was a historical trope copied from Herodotus, and/or Caesar’s Commentarii de bello gallico (Helm 1946: 7-12). The fact that Caesar was talking about Celts, and his description of religion among the Germani mentions worship of the sun, moon, and fire, does nothing to secure the reliability of such ‘historical’ sources. Either way, the argument that the foremost deity interpreted by Tacitus as a ‘Mercury-type’ must be Wodan/Óðinn is a projection of the latter’s supremacy in Old Norse material onto a Germanic society several centuries older. This becomes a circular argument and cannot be leading.

> “taken together, they strongly indicate that Óðinn, although not exactly the same as the god that we know from the Nordic sources, has roots reaching far back in time, probably as early as the Indo-European era

A reader might be interested in Kris Creshaw's "The One-eyed god" which reiterates Odin's similarities with other Indo-european analogues, specifically Apollo and Vedic Rudra.

Indeed, much of the Odin's acts and characteristics find similarities in the deeds of Rudra or Indra in Vedic myth. The Vedics even had a conception of afterlife similar to Valhalla, where the most excellent people and fallen warriors reach Indra's realm, carried by Apsaras.

Indra is also god of sacred verse, "vipratamo kavinam", which is similar to odin mastering the runes, giving a dual priest-king and warrior king function.

The efficacy of "several great deeds" is emphasized for both. Eg: The Havāmal: "140. word following word, I found me words, deed following deed, I wrought deeds.". The Rig Veda (1.101.4) - Praised, he is firm at every deed of his.

Tacitus isn't the only interpretation of the Germanic gods. He is thought to refer to Thor as Hercules, but there are other references where a Jupiter is mentioned. There is a Roman complaint that the Germanic peoples see Mercury as the father of Jupiter when it should be the other way around.

And while it's possible, it would be extremely surprising for Odin to be a new addition to the Germanic pantheon when we find him attested under that name in the 5th/6th century. He's in charge of the whole thing! The norm is for gods - all gods - to have very deep roots. Where we can prove that a god is novel, we can also often show that it's a borrowing of a foreign god with deeper roots (e.g. Adonis < Tammuz) or that it is an explicit deification of a human (e.g. if you go to the temple of the city god in Shanghai, there's an informative plaque explaining that the city god was posthumously appointed to the position by an emperor of the Yuan dynasty).

I do understand that after cassava or maize was introduced somewhere in Africa, anthropologists documented a new goddess associated with the crop. Innovation exists. But pantheons are very conservative overall. "Several centuries" is not an amount of time where we expect to see pantheonic turnover.