In Christopher Marlowe's "The Tragical History of Doctor Faustus", why did Faustus begin to believe that human salvation was impossible?

Based on the story of "The Tragical History of Doctor Faustus", Faustus signed a contract with Mephistophilis, even though few signs appeared before him that he should not agree with the contract. He chose to be on the side of the devil and did a lot of bad things, without thinking what will be of him after the contract. All what he did make him guilty, and gave him the thought that after all what he did, it is too late for him to be forgiven.