When I was growing up, books of mythology were always among my favorites. Often these books would include a list of gods, a “cast of characters” that comprised the pantheon of deities who were featured in these stories. More often than not this list was also the list of deities worshipped among the culture that created the myths.
A pantheon, the way we usually use the term, is the group of gods worshipped by a culture. We can get some idea of a culture’s pantheon from its mythology, but it’s not a complete understanding, and it is often not the same understanding that we get from a study of the religious practice of the culture. Stories are one thing, custom is another. And in either case there can be a certain artificiality to a pantheon. The gods and their interrelationships are not limited by our conceptions of them; myth can help to provide deity with an overlay of humanity that can aid our understanding but it is necessarily an incomplete understanding. What is also true, however, is that a pantheon is not a modern construct but one that has existed for a very long time.
The Greek gods are arguably the best-known of ancient pagan deities and constitute a pantheon well documented by both myth and practice. Even so, their grouping is not as stable as, say, Edith Hamilton would lead us to think.
While there are a great number of Hellenic deities, the dodecatheon or primary group of twelve are those we think of first. In Athens and in many other places these were Zeus, Hera, Demeter, Poseidon, Athena, Aphrodite, Hermes, Artemis, Apollo, Ares, Hephaistos and Hestia (in Athens this grouping is first attested in the 6th century BC, although there is of course no way to know how long it existed before that).
But while the notion of a group of twelve gods was consistent throughout ancient Greece, the specifics could vary regionally. Hestia and/or Ares could have their places occupied by Hades, Dionysos and/or Heracles. In Pherai, a place was held by Themis rather than Hera as consort of Zeus, and another by Enodia who perhaps took the place of Artemis. At Olympia a radically different dodecatheon has been attested, including not only Zeus, Hera, Poseidon, Athena, Hermes, Apollo, Artemis and Dionysos but also Kronos, Rhea, the Charites (Graces) and the river god Alpheios.
(Of course you don’t have to limit your worship of the Greek gods to twelve, and you don’t have to include them in your worship if other less well-known deities call to you more strongly–it just shows how much even communities could vary in how they related to the gods.)
The mythic nature of the gods’ relationships to one another also varied. Aphrodite was the daughter of Zeus and Dione, except that she was also created from the blood of the castrated Ouranos when it fell into the sea and thus an older deity than Zeus himself. Eros was the child of Aphrodite, but he is also a much more ancient and primordial deity, self-created at the beginning of existence.
So while the Greeks clearly perceived their gods as interrelated and as part of an immortal community, the nature of those relationships was variable, and the mythology was neither linear nor consistent.
What we know of the grouping of the Norse gods worshipped in Scandinavia comes mostly from myth, particularly the Elder (Poetic) and Younger (Prose) Eddas. That’s where we find the names of the gods, their attributes and what we know of their relationships. This literary evidence is not always supported by what is known about religious practice, either from historical writings (such as those of Tacitus or Adam of Bremen) or from archaeology. We do know that there was a temple at Uppsala, Sweden where Odin, Thor and Frey were worshipped, so we know that these gods can be said to have comprised a pantheon in practice.
All in all, though, there’s plenty of evidence to support dealing with the Norse gods as a related group. Modern heathens may disagree on who was, is, or should be considered a part of the Norse pantheon, but there is generally agreement on the fact that it existed.
There is also some disagreement on whether certain deities are in fact deities or merely manifestations of other deities. Frigga’s handmaidens (Saga, Eir, Gefjon, Fulla, Sjofn, Lofn, Var, Vor, Syn, Hlin, Snotra, Gna) are considered by some to be aspects of Frigga herself. It’s not a wholly unreasonable thought, since the Norse gods are known by more than one name (Odin in particular has so many that it’s hard to keep track of them all!).
Personally I am inclined to see them as separate, individual deities, partly because several of their number do in fact appear independently, partly because of my own natural polytheistic outlook, and partly because I feel that regardless of whether an entity is “God A Under A Different Name” or “Totally Different God B,” it’s only polite to use the name they provide because there is certainly a reason for it.
(Please note that this section is about the Norse gods specifically, not the Germanic deities in general or as a whole.)
It is not really proper to speak of a “Celtic pantheon.” Just let me get that out there to start–it’s important to be regionally specific about the gods of the Celts, because there are great differences despite their sometimes being etymologically related.
While there is literary/mythological evidence grouping the gods of the insular Celts, this information is very much lacking with regard to the continental Celtic deities.
But let’s begin with what we know of Celtic myth. The gods of Ireland and other Gaelic lands, the Tuatha de Danann (including Brigid, Manannan, the Dagda and their relations), can probably be considered a group in that they were featured in some of the same stories. Likewise the gods of Wales. There is some question as to whether specific entities are deities or lesser beings (heroes and so forth), in part because those who wrote down the stories had a vested interest in this issue, but if you decide a pantheon exists based on whether the gods were perceived as a group, when it comes to the insular Celtic deities it seems that that may well have been so, although without direct historic or archaeological evidence of worship of specific deities there’s a certain level of supposition there as well.
When you get off of the islands (or into the Roman-occupied areas of the isles), however, the evidence for a Celtic pantheon is much less clear. The lack of a surviving mythology means that there is no evidence that most of the gods were seen as interrelated in any way. There are exceptions–according to Tacitus, the gods Esus, Taranis and Teutates were particularly honored among “the Celts.” And there are existing sites where more than one deity received offerings; often a male and female deity who seemed to be “paired” although it is not always clear whether they were considered consorts or were simply worshipped in the same place.
But although some of the continental Celtic deities were more widely worshipped than others (for example, the Matres–mothers–were worshipped over an incredibly broad area), that’s not to say that they comprised a pantheon in the same sense that we know there was a Greek pantheon. Regional and tribal variations existed and the gods worshipped in one place were very often entirely different from those worshipped in another.
So, short version–while it may be reasonable to talk about a Gaelic or Welsh Celtic pantheon, the same does not hold true for the Gaulish deities. You can certainly worship the gods of Gaul, but to call them a pantheon is almost surely inaccurate. It is, for example, unlikely that the British Sulis and the Iberian Endovelicus would have been worshipped side by side in the normal course of things, simply because they are gods who come from vastly different and distant regions.
Please note that I’m not saying that you can’t worship gods from different regions of Gaul. You can. Just recognize that they probably did not have an existing relationship with one another in ancient times.
It’s also good to keep in mind that nearly all of the archaeological evidence that lets us know who these gods were dates to the Roman occupation, the effects of which on the local religion are nearly impossible to know.
Why does this matter?
Whether this matters will depend on your own religious practice. So for you it may or may not matter (although I think it is always interesting to know these things).
I’m writing from the point of view of a multi-faith polytheist who tries to honor the gods in ways that are somewhat familiar to them. I feel like it’s the polite thing to do. I’m not a strict reconstructionist but I do rely on what I know of history to inform my practice. (It makes less of a difference, I find, in day-to-day household practices than it may with regard to larger or seasonal festivals.) That means that I generally try to worship the gods within their own cultural context. As (almost) always, YMMV.