Faustus asks Mephostophilis to bring Helen of Troy to him, to be his love, and Mephostophilis readily agrees.
The devil brings forth the shape of Helen, and leaves. Faustus gives the most famous speech of the play:
Was this the face that launched a thousand ships,
And burnt the topless towers of Ilium?
Sweet Helen, make me immortal with a kiss.
Her lips suck forth my soul: see where it flies.
Come, Helen, come, give me my soul again.
Here will I dwell for heaven is in those lips,
And all is dross that is not Helena. (5.1.97-103)
The Old Man re-enters, watching, as Faustus speaks of how he'll relive the myths of Greece, with Helen as his love and himself playing Paris of Troy. He leaves with her.
The Old Man watches, and knows Faustus is lost. The devils enter, to torture him, but he is completely unshaken. They cannot harm what matters, and he faces them without fear.
The Old Man offers Faustus yet another chance to repent, and makes clear that Faustus can still be saved. But Faustus chooses instead to take a lover-spirit in the shape of Helen of Troy. His speech is beautiful, but as usual Faustus is all talk. He seems unable, or unwilling, to realize that his poetic praise is only a damned man's fantasy. Helen of Troy is not there: Faustus makes love to a dream.
Even within his fantasies, Faustus reveals his failure. Though he fantasizes about being Paris, the Trojan prince who causes the war by abducting Helen, he chooses not to remember that Paris is traditionally depicted as a coward and moral failure. Faustus speaks of battling for Helen: "And I will combat with weak Menelaus, / And wear thy colours on my plumed crest. / Yes, I will wound Achilles in the heel, / And then return to Helen for a kiss" (5.1.106-7). The language is beautiful, but Faustus has altered his source story. Paris did indeed fight Menelaus, but the Greek king was far from "weak." Only the intervention of the gods saved Paris, and by allowing himself to be saved, Paris doomed his city and his people to destruction. Faustus imagines himself as a Greek hero, with a touch of the chivalric lore. His talk of wearing Helen's colors on his crest was a knightly tradition. But shooting Achilles in the heel was not a knightly act. It was an example of weak man beating a far better one, by exploiting a unique weakness. This speech shows Faustus' problem. He seems to know the Greek stories, and loves their beauty, but he doesn't understand them. Though he rejected the Christian God in part because he thought to aspire to Greek greatness, his understanding of the Greek worldview is selective and shallow.