There has been much debate about whether or not Faustus repents in the last hours of his life. Surely, as his end draws near and he can see hell arriving, he regrets the bargain he has made with the devil. He had been regretting it even before the very end, telling Mephistopheles that he was tricked by him. Mephistopheles agrees that he did trick Faustus.
In his last hour of life Faustus says he repents, and even calls out in anguish that even a drop of Jesus's blood could save him. He tries to bargain with God, and he asks God if can simply serve a sentence in hell rather than staying there for all eternity.
Yet while he does not want to go to hell, Faustus does not sincerely repent. First, he has already made up his mind he is doomed, and he goes so far as to reseal his bond with Lucifer shortly before he dies.
If Faustus had truly repented, he would have humbled himself to plead for God's forgiveness, but this he does not do. He asks for more time in his last hour of life, but he has plenty of time to repent: he hardly needs a minute if his repentance is sincere. Yet he says:
Let this hour be but a year, a month, a week, a natural day, that Faustus may repent and save his soul!
Then he both calls out to Christ and Lucifer, as if he will grasp at whatever straw will save him, saying:
One drop of blood will save me: O my Christ!— Rend not my heart for naming of my Christ; yet will I call on him: O, spare me, Lucifer!—
Repentance means a turning away from evil: it means a complete transformation of one's mindset. That Faustus would call out both to Christ and Lucifer means he has not turned away from evil. He is still wavering and simply looking for whatever out he can find. Even as the clock strikes twelve, Faustus does not turn to God but to nature ("air") for salvation, saying:
Now, body, turn to air, or Lucifer will bear thee quick to hell!
Faustus has been so tricked and turned around by the devil that he doesn't know what true repentance is.