Goethe's Faust is a story of a man who is not only displeased with his mortal life but also has lost the concern for his mortal soul as well. Faust is representative of every common man, and God uses him to demonstrate that:
A good man harried in his dark distraction
Can still perceive the ways of righteousness.
Mephistopheles accepts this challenge assured that he could corrupt the soul of Faust through mortal temptation. Despite all of the dark and horrid things that Faust does throughout the play, he always seems to have a moment of dilemma with his conscience prior to his committing these acts usually followed up with a sense of remorse following the deeds. This sense of remorse and moments of conscience are in the end what God says saves Faust's soul and thus leads him to redemption.
When we enter the play, we find Faust as a lonely old man who is on the verge of committing suicide. In fact, one could argue that if Faust had followed through at this point his soul would have been condemned. The reason for this is that Faust had given up the struggle to strive. Up until this point, Faust had always challenged himself to explore new areas of the mortal existence. He tells us in the opening prologue the extent of his life:
I've read alas! Through philosophy,
Medicine and jurisprudence too,
And, to my grief, theology...
And so I've yielded to magic's sway.
This demonstrates that even though Faust may get frustrated with his life he always turned in a new direction to learn more about the world, man's existence, and absolute truth. This continued striving is what makes Faust a good man up until now, and despite his errs, he can still seek salvation.
We can see the differences between Faust and Mephistopheles right from the beginning. When Mephistopheles makes the offer, Faust quickly amends the deal saying:
If you can cozen me with lies
Into self-complacency,
Or can beguile...