As Noah departs the Ark after the Great Flood, God extends the general dietary principle, what He’s classified as food. In the Garden, He had revealed his provision of plants for nourishment (Ge 1:30), and now He’s allowing anything that moves to be eaten. (Ge 9:3)
What’s interesting to observe about God’s dietary revelation is that it’s very general; important, critical details are omitted. For example, we shouldn’t eat certain kinds of plants because they’re poisonous, yet God never explicitly tells us which plants to avoid and why.
As a general principle, plants are food, but each particular animal species should only eat certain kinds of plants. God gives each species instincts about what’s good to eat, and places Adam in a special garden stocked with a wide variety of edible herbs and fruit trees as a start. He also gives Man intelligence to figure out the rest, so with a bit of trial and error, we do just fine in the antediluvian world. The key point being this: just because something appears to be in the general category of food, doesn’t mean that we humans should be eating it. We need wisdom and discernment to be healthy.
The same appears to be true for eating animals; this dietary extension to eat flesh applies to certain animals as well as to humans, according to God’s design in each of His creatures. By nature, some creatures are merely herbivores and some are capable of being carnivores or omnivores. So, as Noah considers God’s extension of the dietary principle to include meat, as there’s a design apparent in certain animals that enables them to eat it, there’s also an obvious guideline for Man about which animals are good to eat, which Noah understands to be clean.
This concept of clean animals wasn’t new, it was well-known in the antediluvian world, even though we’ve no record of any direct revelation from God about it. Perhaps Adam discerned that certain kinds of animals were distinctly different from others in a way that made them suitable for humans to domesticate, even though we weren’t eating them. For example, Adam might have discovered that milk and wool from sheep were especially good, and taught his sons about it. Perhaps this is why Abel chose shepherding as his profession. (Ge 4:2)
In other words, Adam had not merely named all of the animals (Ge 2:19), but he may have observed enough about each species to classify it as clean or unclean, and taught the rest of us how to distinguish between them. Perhaps this is why, when God told Noah to take into the Ark seven of each of the clean species of animals, and only two of each unclean species, He didn’t need to explain; Noah appears to have already known exactly how to do this. (Ge 7:2)
And as Noah is leaving the Ark, contemplating the spare of each of the clean animals, he perceives that God will be pleased with an enormous sacrifice (Ge 8:20), an expression of God’s ownership of all things, rejoicing in His pleasure in sparing life on the earth.
After the sacrifice, noting the remaining three pair of each clean animal species, and only one pair of all of the other animal species, as Noah was considering God’s expanded dietary principle, recognizing that eating any of the unclean animals in the near future would cause that species to become extinct, it was immediately clear which animals God intended for human consumption: the clean ones, especially those which we were already in the habit of directly managing.
But over time, this knowledge about which kinds of animals were good for us to eat seems to have deteriorated to the point that it was appropriate for God formally define it for us; as men began to rebel against God in every conceivable manner, the dietary principle was evidently no exception. So, in formalizing His perfect ways for Israel, God reminds them to not eat abominable things (De 14:3), animals which He has not designed for humans to eat, clearly explaining exactly how to distinguish between clean and unclean animals (6) and giving us a number of specific examples of clean beasts (4) and unclean ones (7), edible fish (9-10) and unclean birds. (11-18) This wasn’t a change in the dietary revelation, or even a new concept, just a formalization of what He had already informally revealed in us to establish clarity and accountability.
It is true that there is some nutritional value in unclean foods, but this should not be our preference when we have any kind of choice. Under normal circumstances, routinely choosing to eat unclean animals is to disrespect and harm ourselves. As God has progressively revealed His eternal ways over time, He hasn’t ever changed His mind about what’s good for us, nor has He been arbitrary in His commands: they’re righteous and very faithful, each and every one of them. (Ps 119:138)