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:

readability="7">

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.