While it's not obvious, religion is the foundation on which this poem is built. Dickinson has modified the story of Adam, Eve, and the serpent (a.k.a. Satan, Lucifer, Beelzebub, Old Scratch, Mephistopheles—you know, the devil). “A narrow Fellow in the Grass” draws specifically from the Book of Genesis, the first part of the Old Testament in the Bible. As the scripture goes, Eve was convinced to eat the forbidden fruit from the Tree of Knowledge, and Adam eats it too. After this, Adam and Eve become ashamed of their nakedness and become mortal. Dickinson does a lot in this poem, but the allusion to Adam, Eve, and the serpent is the biblical bedrock upon which everything else in the poem rests.
The curse on Eve and the serpent is revised in the poem to restore the Garden of Eden peace between humans and the natural world. A happy ending after all!
In “A narrow Fellow in the Grass,” the speaker's love for Nature’s People is a reminder of the perfect state once known by Adam and Eve in the Garden of Eden. That was pre-serpent, though, which is why the speaker is so freaked out at the end of the poem.