Directly following Gandalf’s explanation of the Seen and the Unseen worlds to Frodo, Tolkien records for us a prophetic and mystifying rumination on the part of Gandalf. To Gandalf’s eyes, which see in both worlds as the Elves do, Frodo appears slightly transparent. Importantly, the word is transparent and not faded. Gandalf does not think that Frodo will come to evil. But, he reasons, “He may become like a glass filled with a clear light for eyes to see that can.”
The image of a glass filled with light instantly recalls the Phial of Galadriel, which will come to Frodo’s aid so often through his later adventures. The implication of Gandalf’s rumination is clear. Frodo too has begun to live in the world of both the Seen and the Unseen, and to those like Gandalf who can see the Unseen, he is already being transformed into a vessel or medium of invisible virtues. As the Phial of Galadriel brings light to dark places, Frodo himself will presumably come to exude (for lack of a better word) a “spiritual” light in the darkness.
Perhaps it would not be too much of a stretch to take a cue from the Catholics on this point. If Frodo’s sufferings transform him into a phial of light to his world—a vehicle of divine grace, as it were—then this lowly Hobbit of the Shire is destined to become a Saint of Middle Earth.
The Red Book of Westmarch 