The One remains, the many change and pass; Heaven's light forever shines, Earth's shadows fly; Life, like a dome of many-colour'd glass, Stains the white radiance of Eternity, Until Death tramples it to fragments.—Die, If thou wouldst be with that which thou dost seek! Follow where all is fled!—Rome's azure sky, Flowers, ruins, statues, music, words, are weak The glory they transfuse with fitting truth to speak.
Wherever Adonais-Keats is, the "One" is also there. Our speaker's referring to some type of deity, but he doesn't get specific about who this deity is. Instead, he talks more about the heavens. Everything else changes and passes away, but Heaven's light "forever shines" on Earth.
Life, he says in another simile, is like a "dome of many-colour'd glass." (Picture a snow globe.)
The colors of the glass shine through eternity, but then Death comes and breaks it into pieces (more personification here). Death disrupts Life's influence on eternity.
So, the speaker says, if the mourners are unable to live without the youth, they should go ahead and join them: "be with that which thou dost seek!"
Is Shelley talking to himself here? After all, he's also mourning Keats. Hmm.
The speaker says that the beauty of Rome is no match for the beauty of the afterlife, in his opinion. He calls Rome's beautiful objects "weak," compared to the glory they are filled with—which comes from truth.