Understand, I take these words literally. To "understand" something is to literally "stand underneath it." Again, knowledge can only work via the past. It categorizes, analyzes, reduces, and confines. In fact, without standing beneath some principle that we can all agree on, no collective knowledge is possible.
However, like Urizen, the scientists want to control the whole game by asserting the supremacy of their own methodology over all others. A knowledge, a scienz which only sees backward in time can never tell the whole story.
Now let's watch Blake violate this principle.
The ancient Poets animated all sensible objects with Gods or Geniuses, calling them by the names and adorning them with the properties of woods, rivers, mountains, lakes, cities, nations, and whatever their enlarged & numerous senses could percieve.
And particularly they studied the genius of each city & country, placing it under its mental deity;
Till a system was formed, which some took advantage of & enslav'd the vulgar by attempting to realize or abstract the mental deities from their objects: thus began Priesthood;
Choosing forms of worship from poetic tales.
And at length they pronounc'd that the Gods had order'd such things.
Thus men forgot that All deities reside in the human breast.
This is a valid mythographical framework which is seldom seen in the light of a new science. The Genius, or the Poetic Genius could be seen as a rather haughty expression of self-reference, but the Genius is a term used in mythography to describe some force of nature which has been animated, or enlivened, or inspired (literally to in-wind) is Blake's key metaphor for talking about myth which why I say Blake's mythology is so close to Native American spirituality which Blake had a ton of respect for.
So, Blake is arguing that religion is a form of animation, or animism, which has been abstracted from its original source, so that some men/women can become the sole arbiters of the craft.
Blake sounds downright atheistic at this point. If the gods do not literally exist, but "reside in the human breast" then what is it that we're talking about when we invoke the name of God.
Well, we are talking about a principle which has mapped itself onto human consciousness in the form of a deity, a creative deity, a creator, and so the creative principle of man's genius is no less than God himself.
So the key motifs in Blake's mythographical system (as articulated here) are Animation, and Concretization.