This sentence shows that the narrator has an idealized and spiritualized view of romantic love.
A chalice has several associations. First, it alludes to Catholicism. A chalice is a sacred vessel that holds the communion wine—the mystical blood of Jesus Christ—suggesting that the narrator conceives of his love for Mangan's sister as spiritual and ethereal. It also indicates he thinks of this love as a special charge laid on him: he has to protect this precious vessel of his love. He sets it apart from the sordid, everyday Dublin world in which he lives, which he describes as dirty and coarse.
Chalices—and a "throng of foes"—also connect the narrator's conception of love to Medieval romances, especially Arthurian legends and a search for the Holy Grail. The narrator seems to imagine himself as a knight charged with safeguarding a pure love from his enemies.
What little we see of Mangan's sister shows her to be an ordinary Dublin girl, but the narrator needs a love object that is elevated, pure, and set apart from the ordinary world. His concept of love shows his deep yearning for something more exalted than his dull everyday world.