Mephistopheles will be allowed to try to tempt Faust away from the path of goodness. If he fails he has to admit that mankind isn't as wretched as he thought. Faust changes he deal to a bet that nothing Mephistopheles can give him will make him content and fulfilled. Want, Debt, Care, and Need.
Care is the only one who can get in.
In revenge she blinds him. However, he does not gain salvation without the intercession of Gretchen's prayers and God's grace.