What is the central idea of the poem?
The central idea of "God's Grandeur" is expressed in three parts--a sketch of God's character, a lament and a psalm-like praise--and sets forth Hopkins' view that God's grandeur is apparent in the natural world and that, while humankind has smeared and trodden the natural world underfoot, a new day will nonetheless dawn because the world is watched over by the "Holy Ghost," currently more commonly called God's Holy Spirit.
Hopkins uses several metaphors, which compare God to elements in the natural world (i.e., electricity, precious metal, oil), to sketch God's character. Beginning with the grandeur and power of God, he compares God to electricity, saying the world is "charged" with God's grandeur, i.e., with his exalted awesomeness. He then says God sparkles, he "will flame out," like gold or silver foil that is shaken, while also comparing God to fresh oil that collects and builds while newly pressed or "crushed." Yet, he implores, why doesn't humankind pay heed to or pay attention to or "reck his rod." "Rod" is an interesting choice of word because it means both a symbol of authority and power and a tool for administering punishment: If they will ignore God's might, grandeur and power in the natural world, why will they not heed his powerful rod of punishment?
In his lament, Hopkins lists the assaults against the natural world committed by humankind: they "trod," which means they crush underfoot; they burn, searing the world with business and commerce, or "trade"; they make everything indistinct, "bleared," and overspread with goo, "smeared," through human "toil"; they "smudge" the "soil" with their dirty markings and their lingering "smell." The result of these abuses is that "the soil / is bare now" and that humankind cannot feel the soil anyway because they wear shoes--"being shod"--that form a separating wall between feeling and soil.
In Hopkins' psalm-like praise, God's grandeur is shown to be triumphant because the "Holy Ghost" intercedes and delivers God's grandeur. Hopkins starts his praise with the resilience of nature, which is "never spent," never depleted, then describes the uniqueness of each thing in nature when he alludes to his philosophy of "inscape." "Inscape" is an epiphanic vision of the inner landscape of the essential qualities of a thing: the qualities that give the thing its uniqueness. This inner landscape of essential uniqueness is suggested in the line, "There lives the dearest freshness deep down things." He then turns to the "never spent" sunset in a "black West" and to the following sunrise at "morning" that "springs," rising "eastward," above the "brown brink" of a dirty, trodden world.
Punctuation is very important to the meanings Hopkins expresses, and the punctuation here, the line-end em-dash, "Oh, morning, at the brown brink eastward, springs — " leads to the reason why "nature is never spent," as he illustrates with the sunrise that comes from the "black West" night to illuminate the "brown" eastern horizon. Hopkins explains why by employing an effective nesting bird metaphor in which he says the reason nature is never depleted is that the Holy Ghost "broods" and watches over, protects, and warms the "bent / World" with His "warm breast" and His "bright" luminous wings that shine with holy radiance, a radiance reflecting God's grandeur.
Thus, starting with God's grandeur, Hopkins brings us from unheeding humankind to the world crushed by carelessness to a "never spent" natural world that is kept from depletion, despite searing and smudging, by the "brooding" protection of God's "Holy Ghost" whose (metaphorical) "bright wings" display the radiance of God's grandeur.