In Ralph Waldo Emerson's Nature, the poet praises the natural world. Nature is Emerson's first book, and his love and respect for nature and the guidance he believes mankind can find in the outdoors are common themes in many of his works.
Emerson stresses "the harmony between humans and nature." In drawing forth this theme, Emerson speaks about "reason and faith" in describing what one might find in the midst of the glories of plants, earth and sky.
Within these plantations of God, a decorum and sanctity reign, a perennial festival is dressed, and the guest sees not how he should tire of them in a thousand years. In the woods, we return to reason and faith. There I feel that nothing can befall me in life...
Plantations are large agricultural endeavors, either farms or estates, where the principle industry is growing crops in large number. The word can also simply refer to large areas of planted trees, etc. Emerson is using "plantation" to convey to the reader a landscape of plenty, of splendid greenery. The author is specifically speaking about the woods. He describes it as a place of decorum (dignified behavior) and sanctity (holiness). These words clearly convey Emerson's respect and admiration for nature, as he personifies the lush growth as if it were a human thing capable of dignity and godliness. (We can infer that these things are lacking in the civilized world from which the author seems to have escaped. We can also find irony in Emerson's perception that the wilderness offers a more ordered existence than society.)
"Perennial festival" is Emerson's way of saying that the foliage of the trees is a garment (dressed) worn for a celebration—one that returns yearly. Emerson says it is so magnificent that an individual wouldn't tire of being in such a place, not for a thousand years.
Emerson says that wondrous things are found in the woods: it is there that mankind (we) can be sane (reason). The use of faith here most likely refers to communion with God: that the author feels closer to God in the woods. Faith may also lead the reader to believe that nature provides mankind with hope (a word often associated with faith).
At the end of the section, Emerson has not only praised the lush and lovely woods, but he announces that in addition to everything else, he feels safe. (This is also ironic, for one might generally believe that safety would not be found in untamed nature, but in civilized society.)