The Transcendentalist movement (of which many see Emerson as the unofficial "leader") was about religious perceptions "rooted in the ideas of American democracy," a response by a group of Bostonian clergy (including Emerson) who were convinced that the Unitarian Church had become too "conservative." In light of this, they adopted a new theological viewpoint...
...one which privileged the inherent wisdom in the human soul over church doctrine and law.
In essence, this new idea they espoused held the sanctity of the soul over the man-made theological canon. The truth found in one's soul went beyond all that was taught (or enforced...) in organized religion.
In Nature, Emerson writes:
There I feel that nothing can befall me in life, -- no disgrace, no calamity...which nature cannot repair.
His belief is that nature can fix all things. Nature, we can infer, is more powerful than mankind and civilization. Nature is on the same level as God—perhaps we can even assume that nature allows man to be one with God. The idea of transcendentalists is that mankind can transcend the shackles of society, even of the physical, rising to a more intellectual and spiritual plane. Emerson then writes:
Standing on the bare ground, -- my head bathed by the blithe air, and uplifted into infinite space, -- all mean egotism vanishes. I become a transparent eye-ball; I am nothing; I see all; the currents of the Universal Being circulate through me; I am part or particle of God.
Here the author notes that "self" disappears: in its place is a transparency—truth. His physical self is "nothing," but beyond that, he is everything. (This is a paradoxical truth.) In this state, the author can "see all" and becomes one with the world, for he is now a part of God. For the transcendentalist, this would have been the ideal condition to be in: freed from the restraints of the world and drawn into the freedom and wonder of being a part of God in his great universe, "transcending" the elements of the world that force one's soul to be earth-bound.
In "The Poet," Emerson addresses poetry, but not something that is simply a literary exercise. For him it was more than just writing: it reflected his ideas of the world and the concept of spirituality. While some poetry was (as he saw it) nothing more than light entertainment, for Emerson it went far beyond that...it was not even comparable. His poetry, perhaps poetry in its truest and most valuable form, is nothing common, but expresses a uniqueness, a non-conformity. Poetry written simply as an art form, he noted...
...[kept] both writers and their readers "at a safe distance from their experience."
In Nature, Emerson describes an interaction with the world on a spiritual level, where (in a religious sense) poetry lifts one up into the realm of the Almighty. In "The Poet," Emerson describes "true poetry"—the opposite of "unauthentic poetry"..., which also causes the dampening of the spirit. "True poetry" uplifts the soul, and all human beings have much to gain in the pursuit of this kind of writing (transcending physical boundaries), and much to lose in ignoring it (fettered to the physical).
Emerson's underlying claim is that experience has a spiritual dimension that poetry can reveal.
This spiritualism is alluded to in the quote above from Nature, and the theme continues in "The Poet," reiterating the transformation of one's existence, one's soul, enabled by "true poetry."