Faust's sin was that he sought "with ecstasy, / To rank itself with us, the spirits, heaved." Trace Faust's career as a sinner in Faust.
This is a very interesting question because Goethe does not really paint Faust in Faust Part I as a sinner the way Marlowe paints Faustus as a sinner: Faustus intentionally conjures Mephistophilis ("And try if devils will obey [my] hest, / ... / And try the uttermost magic can perform") with the express purpose of gaining power to break the laws of nature ("Be it to make the moon drop from her sphere, / Or the ocean to overwhelm the world") if so desired (the limit Faustus finds is that Mephisto answers to Lucifer, not to Faustus!). In contrast, Faust implores the powers of the cosmos in order to know the knowledge that unifies ("Secrets now veiled to bring to light / ... / That binds creation's inmost energies;"). He is sent Mephistopheles when the cosmos finds Faust weak and insignificant!: "in his spirit's depths affrighted, / Trembles, a crush'd and writhing worm!"). Faust's sin is that he sought "with ecstasy, / To rank itself with us, the spirits, heaved."
So while Faustus is an intentional sinner striking an accord with the Devil through his minion Mephistophilis, Faust is an accidental sinner who is lured into a skeptic’s wager (a doubter's wager: he doubts what Mephisto says) with Mephistopheles. The extent of Faust's characterization as a sinner will be limited by what the Lord says in "Prologue in Heaven":
A good man in his darkest aberration,
Of the right path is conscious still.
This means that The Lord asserts that Faust (1) is a good man who (2) may walk in the "dark" side of life but who (3) still has a consciousness of the right path and therefore (4) will still ultimately choose the right path--the path of light--in the end. This pronouncement by The Lord is very important to understanding Faust's characterization as a "sinner" as well as his final salvation and ascension into Heaven.
The steps in Faust's sin include the important milestones of his encounter with Helen of Troy in the “Witches' Kitchen” and subsequent encounter with Gretchen. Faust, who has never given time or energy to pursuing human passions, is deeply enamored of Helen when he sees her in the magic mirror. It is this feeling that allows Mephisto to finally convince Faust to drink the magic love and youth potion; it is in turn the potion that makes Faust lust so earnestly after Gretchen--whom Mephisto advises against but whom Faust desires as being reminiscent in some way of Helen. This lust, fueled by Mephisto's evil genius, leads to seduction, a devil's trick that slays Gretchen's mother, and an execution for the double murder of her mother and her infant.
Another milestone is the duel Faust fights with Valentine, Gretchen's brother, while under the influence of Mephisto's control:
Doctor, stand fast! your strength collect!
Be prompt, and do as I direct.
Since Faust slays Valentine--albeit while Mephisto has control of him--he must flee the city, which was part of Mephisto's corrupt intention once the fight began: MEPHISTO: "I confess, / A touch of thievish joy and wantonness." As a consequence, since Faust is away from the city in the Hartz Mountains and then in a Plain, he has no way of knowing Gretchen's suffering. Faust Part II takes a dramatic turn and tells the rest of the story in a Classical style since Goethe rejected Romanticism, therefore Faust's characterization changes and he proceeds on a Classical quest for universal knowledge as is represented by the Homunculus.