This is one of William Blake's most famous poems. It starts with the lines everybody knows.
To see a world in a grain of sand
And a heaven in a wild flower,
Hold infinity in the palm of your hand,
And eternity in an hour.
Yes, these are so well-known they are now almost hackneyed. They do capture the feeling that arises in a typical peak experience or transcendental state of consciousness, but that state can come to almost anyone seemingly at random, and even be stimulated (or perhaps simulated would be a better word) by drugs, and, though it may be called spiritual, it cannot be said to represent the spiritual goal at which we should be aiming. It is perhaps the goal of pantheistic mysticism but it is not the theistic goal.
That is revealed later with the last lines of the poem. Here they are.
God appears, and God is light,
To those poor souls who dwell in night;
But does a human form display
To those who dwell in realms of day.
With these words Blake draws a clear distinction between two different kinds of spiritual approach, and he comes out in favour of the latter. The first approach is that of the philosopher who seeks knowledge or the Eastern mystic who wishes to dissolve his (supposedly) illusionary self into the greater Self for it there to disappear into the unmanifest ocean of light. This is a real state but to see it as our spiritual goal totally ignores the whole purpose of creation. If God is light then he is abstract and impersonal, and creation, ultimately, is unreal. Nothing is real except the state of blissful unmanifest oneness. Now, reality does have an aspect in which there is simply pure light but that is not the whole of what reality is or what God in himself is, and it is not the aspect that we were created to fall back into. For it is rather like returning to the spiritual womb before we experienced this world of creation. But we were born, and we are meant to grow into sons and daughters of God, real individuals with real creative powers who can expand creation through love.
When Blake says that God has a human form he means that God is a person. He is saying that reality is personal and made up of beings not things, concrete not abstract. This is deeply unfashionable today but it is the truth taught by Jesus who stands for us as the human form of God, and to know it fulfils our spiritual purpose in a way that being reabsorbed by the light can never do.
Note that Blake says God is light to those who dwell in night. To say that God is light might seem to be the highest thing one can meaningfully say about him, but Blake says that this is the case only for those who live in ignorance, whose spiritual darkness tells them that God must be light. Real seekers but not yet awake. However, to those souls who have risen from their spiritual slumbers and "dwell in realms of day", God is not just light. He is a person. Reality is personal, and that means that our individuality is a real thing. A thing given by God, self-evidently, but what God gives he does not take back. God is creative and he wants us to be too since we are his children. To be creative, you must have a self.
William Blake was probably the pre-eminent prophet of Albion. If I may interject a personal note which I may have mentioned before on this blog, when I was 16 I developed a real enthusiasm for him and asked for and received the complete works of his poetry for Christmas. I was also a regular visitor to the Tate Gallery in London which had (and presumably still does have) a large collection of his paintings. To tell the truth, I was rather daunted by his longer poems and never read any of them all the way through. I certainly didn't understand what I did read. But I loved the Songs of Innocence and Experience, and found his pictures fascinating, partly, I think, because they depicted visions of a higher world of endless creative energy produced by someone who actually seemed to have seen what he was painting. Artistically, they are somewhat crude but the inspiration comes through and bursts off the paper or wood or whatever the material might be. Blake was not an orthodox Christian because his natural insight was too great to be confined by any framework of systemised thought. That doesn't mean he was right in everything he said or wrote, but he was undeniably authentic so, even when he might have been in error, I would say that was because the over-flowing abundance of his vision exceeded his mental capacity to express it.
In modern times even those who do turn to a spiritual conception of the world are often more comfortable with abstract explanations of the God is light variety. A personal God just seems too childish. We've evolved beyond that anthropomorphic way of looking at things. We are now much more intellectually aware. But the funny thing about real evolution is that we often return to where we were before, but see it this time in a deeper way. God as person is not childish. It is real in a way that philosophical approaches to God can never be. Blake's vision in the Auguries of Innocence was true.