In Christopher Marlowe's The Tragical History of Doctor Faustus, the first step is to identify the climax. This is sometimes a tricky business, for some readers perceive the climax to be in one place while others see it somewhere else.
For me, the climax of the play comes toward the end when there still seems to be some question as to whether or not Faustus will repent and save his soul or not. There certainly seems to have been some concern on the part of Mephisopheles and the "powers of darkness," while the Good Angel and the scholars tried to convince Faustus that all is not lost—that there is still time to save his soul. All along Mesphistopheles and those with him have done all they can to distract Faustus when an opening to get out of the contract with the Devil has presented itself.
In Act Four, scene four, Faustus contemplates redemption: he remembers the thief on the side of Jesus at the crucifixion, and recalls that he was saved, but the thief repented of his sins, which Faustus does not do—instead he falls asleep. This is the first inkling that a reckoning is close at hand for the "magician." Perhaps Faustus just does not care enough. When he has the power and opportunity to reverse his fate with a few words and a "right heart," he falls asleep.
In Act V, scene one, an Old Man appears and encourages Faustus to repent.
This would seem to me to be the climax: when all hangs in the balance, when it seems as if the Devil could ultimately lose Faustus' soul after all this. In learning the error of his ways, Faustus might still be able to go on with his life while looking forward to eternity with his sins forgiven. This is not, however, the case.
Even at the Old Man's urging, Faustus believes that there is no hope for him—that he cannot turn back because the Devil already owns his soul. By asking Helen of Troy to make him immortal, he gives away his last chance at redemption because he has not asked God instead.
The rest of the play is the "falling action" that moves toward the resolution of the plot. At the start of Act Five, scene two, Lucifer, Beelzebub and Mephistopheles arrive to take "possession" of Faustus' soul. Faustus does not repent.
The Good Angel appears again—this time to tell Faustus that he has missed his chance to redeem himself and his soul, and the Bad Angel also appears—to gloat. Still Faustus ignores the chance to plead for God's forgiveness.
Between 11:00 and 12 midnight, Faustus expresses his regrets. However, while he believes there is no hope, he does not even try—on the off chance—to repent. An ever-loving God might still hear his cry for help. He says nothig, and at the stroke of twelve, Mephistopheles takes Faustus with him, collecting Faustus' debt to the Devil.