The key irony in the play Doctor Faustus by Christopher Marlowe is that the soul of leading character- Doctor Faustus is in the hold of the same devil who so-called claims to be at his total, 24X7 service. Faustus thinks he is the master of the world and takes pride in the grand magical powers he has acquired, but his status is actually diametrically opposite. He has not achieved anything grand but is about to lose whatever he has got. Signing the pact with devil will not lead to any happiness or blessing but is just his impending damnation.
Besides that Faustus' interpretations of Biblical text are ironical. Consider the following lines from scene 1:
The reward of sin is death. That's hard...
...If we say that we have no sin, we deceive ourselves,
and there's no truth in us. Why then, belike we
must sin, and so consequently die.
Faustus missed the significance of repentance and forgiving power of God. The whole of the play's focus is the divine knowledge Faustus wants to attain but, in true sense, his knowledge and understanding is poor, hollow and worthless.
The greatest irony in Dr. Faustus is his hubris(excessive pride), and his faith in his own mortal powers to command the service of devils. in the end, he is torn limb from limb, and his soul is cast into the doom of Hell through his unwise decisions.
Structural Irony is present throughout Faustus as a whole. Marlowe presents this structural irony through the traditional use of an unreliable or naive narrator: Dr. Faustus is smart, ambitious and proud of his accomplishments, but refuses to recognize the faulty reasoning which eventually leads to the loss of his soul. Upon graduation, he rejects professions in law, medicine, theology and philosophy, believing that he is destined for something infinitely more satisfying, such as an omnipotent destiny filled with power and worship by the masses. This structural irony is supported by different types of irony such as dramatic and situational irony throughout the play.
An example of situational irony occurs when Faustus summons Mephistopheles, a demon who is servant to Lucifer. In exchange for twenty four years of power, honor and earthly riches, Faustus aims to sell his soul to Lucifer. The Good Angel attempts to dissuade him from such a disastrous course of action, but the Bad Angel tells him that all his fears are just inconsequential nuisances in his quest for self-fulfillment. Faustus falsely believes that he will never be called to account for his deadly decision as he believes that hell is a myth; however, we readers know that the Good Angel is actually right.
When Faustus asks for a wife after he seals the deal with his own blood, the devils tell him that the option of holy matrimony is now beyond his reach. Instead, they send him a succession of prostitutes for his sexual enjoyment. In the meantime, Beelzebub, Lucifer and Mephistopheles entertain him with absurd and entertaining manifestations of the seven deadly sins. This is dramatic irony, where the reader knows something the character does not seem to be aware of: the reader knows that the devils want to lure Faustus into a false sense of security. Another example of dramatic irony is when Faustus continually questions Mephistopheles about the universe and the primary concept of life but is constantly rebuffed with the admonishment 'to think on hell.' We know this is another way to lull Faustus into deeper and deeper indiscretions.
Another example of situational irony comes in the final stages of the play when Faustus gives way to despair instead of repenting. He begs for more time from the devils and asks Mephistopheles to summon Helen of Troy for the scholars, thinking that this will buy him the time he needs. He would rather have a dalliance with fantastically conjured up images than repent of his sins. In the end, his fate is sealed: two scholars eventually find his horribly mutilated and dismembered body. He loses his soul to Lucifer.