Nothing gold can stay. (Title, 8)
If the speaker of the poem believes in heaven, we hope this title is only referring to life on earth. When the poem refers to the Garden of Eden, it's important to remember that, when Adam and Eve sinned, they were still offered the far-off promise of redemption, an eternal paradise in heaven. Now that, the Bible suggests, is something gold that can stay.
Nature's first green is gold (1)
The speaker refers to nature throughout the poem. It seems like there's something spiritually rewarding for him about the brilliance of nature, as transient as its spring beauty may be.
So Eden sank to grief, (6)
The speaker brings Eden into this super short poem, which means that if he's not religious himself, he's at least aware of the significance of religion in his reader's consciousness. Even though, as this poem shows, paradises of many kinds are fleeting, something in the human heart seems to always want to keep looking. Perhaps, just as this poem sticks with us even though spring blooms can't, the hope for something better is another kind of gold that can stay.