By this point, it should go without saying that the Norse thought of their gods as highly anthropomorphic beings – that is, they were very much like humans, just writ large. Even their spiritual nature didn’t separate them absolutely from humans, or, for that matter, from the rest of the physical world. Just as humans had a material part and a spiritual part or parts, the gods, though spiritual, manifested themselves in various physical phenomena. In the lingo of philosophy of religion, this is called a “theophany” (the manifestation of a god) or a “hierophany” (the manifestation of the sacred).
For example, Thor, whose very name meant “Thunder,” was not so much the “god of thunder” as he was the god thunder – the divinity whose presence the Vikings felt in the thunder. His wife, the goddess Sif, was known for her long, luscious, golden hair that seems to have symbolized fields of ripe grain. Sif would therefore have been the goddess grain, and the storms fertilizing the vegetation would have been practically a ritual enactment of the consummation of the marriage of Thor and Sif.
This wasn’t exactly pantheism, the idea that all of nature or the physical world is divine. There’s no indication that the Norse thought that the physical world in its entirety manifested the gods. But parts of the physical world certainly were thought to embody them. (It’s extremely doubtful that there was ever a firm list of which parts did so; the Norse seem to have treated this as more of an “I know it when I see it” kind of thing.)
Since the gods were imagined to have human characteristics, and since they regularly manifested themselves in, and intervened in, the affairs of the world, it was possible for humans and gods to interact with each other. Such interactions were an essential part of Norse religiosity.
This occurred in countless different ways, the most intimate of which was the belief that the gods copulated with humans in order to found royal and heroic families.
The most common interaction between the gods and humans happened through ritual sacrifice, the cornerstone of Norse religious practice. The pragmatically-minded Norse didn’t only worship their gods out of a sense of wonder or love. They also usually wanted to get something in particular from the gods.
In human interactions, if you want to get something from someone – at least in a way that maintains a healthy relationship between the two of you – you have to give that person something in return. Since the gods were so much like humans, when humans wanted something from the gods, they had to give them something of value, too. This was the logic of sacrifice: by piously offering the gods a gift, their human worshipers hoped to receive gifts from them.
This reciprocity between the gifts of the gods and humans mirrored the more strictly human reciprocity between a Viking warrior and his chieftain. The warrior who fought bravely and loyally for his chieftain would be rewarded with his share of whatever spoils were taken in the battle or raid. Despite the unequal status between the warrior and his chieftain, and the unequal status between humans and gods, both parties in these transactions had obligations to the other that they were expected to fulfill. The warrior had obligations to his chieftain, who in turn had obligations to him; and humans had obligations to the gods, but the gods in turn had obligations to them. When human worshipers performed the appropriate sacrifices, they could legitimately expect the gods to bless them with victory in battle, bountiful harvests, sexual fertility, or whatever it was they sought.
There was an element of unconditional fealty present in the chieftain-warrior relationship as well, exemplified by the expectation that an honorable warrior would sooner die by his chieftain’s side than flee and live. But this was largely subsumed by the sense of mutual obligation; a Viking warrior could choose to whom he offered his mortal loyalty, and leave one chieftain for another if he thought that another would treat him with more generosity.
As chieftains became kings and Christianity triumphed in the later part of the Viking Age, the emphasis was reversed. The relationship between the king and his fighters – which had necessarily become much more impersonal with the great increase in the number of fighters each king commanded – was spoken of in terms borrowed from Christian language. No longer did humans and gods have reciprocal obligations to one another, in which both parties participated more or less voluntarily and held a dignified position despite their immense inequality. In the same way that medieval Christians were supposed to serve God unconditionally as his “slaves and thralls,” so, too, were a king’s men supposed to serve him. What had previously been a contract or a bargain was replaced with decree, fiat, commandment.