In Goethe's Faust, why does God allow Mephistopheles to tempt Faust?

In Goethe's Faust, in the "Prologue
in Heaven," the Lord is in heaven, gathered with his heavenly host. Included among those
there are the angels, Raphael, Gabriel and Michael; Mephistopheles is also
present.

Each of the angels praises The Lord's creation,
including the sun, the oceans and even the power of thunder and lightning storms. Since
the primal (first) day of the world, all of The Lord's good works are as glorious and
beautiful as they were on that first day.

Mephistopheles
(Mephisto), claims that he has no words of praise for the sun or the ocean. He
complains, instead, that since mankind saw the first glimmer of heaven in the knowledge
they received in the Garden of Eden, they have been
miserable.

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You’ll get no word of suns and worlds from
me.

How men torment themselves is all I
see.

The little god of Earth sticks to the same old
way,

And is as strange as on that very first
day.

He might appreciate life a little more: he
might,

If you hadn’t lent him a gleam of Heavenly
light:

He calls it Reason, but only uses
it

To be more a beast than any beast as
yet.

Mephisto's complaint is
that rather than rising above the animals with that "gleam of Heavenly light," they act
more like beasts every day. Mephisto, therefore, has no praise of The Lord or his works
at all, only criticism.

The Lord asks whether he comes
always "thus, with ill intention?" Does nothing go "right on earth"? But Mephistopheles
complains that it is so bad, not even he wants to visit there
anymore.

The Lord has great faith in one man in particular,
and so he draws Mephisto's attention to Doctor Faust. (This part of the plot is very
similar to Satan's temptation of Job in the Old Testament of the Bible.) The Lord says
he may be a doctor, but he is The Lord's servant first. To prove that Mephisto is wrong
about mankind, The Lord permits Mephisto to tempt Faust as long as he is alive. The Lord
is certain, though, that by the end of his life, he will still be his faithful servant.
Faust may be having doubts now and making mistakes, but he will come around: as long as
he lives, at least he is still trying: mistakes are part of
living.

For
while man strives he
errs.

So, in essence, the
temptation of Faust is Goethe's description—in this literary piece—of how man is tempted
every day of his life, choosing between The Lord and "darker powers." In this piece,
Goethe presents the situation as a wager between The Lord and Mephistopheles. The Lord
does not doubt Faust, and the wager is on so The Lord can convince Mephisto that
humankind is inherently good, despite the temptations that come his way; The Lord
believes Faust will remain faithful and learn some important lessons on the way. The
Lord predicts what Mephistopheles will find:

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And when you must, then stand,
amazed:

A good man, in his darkest
yearning,

Is still aware of virtue’s
ways.

When Heaven closes,
Mephistopheles comments on what a gentleman The Lord
is:

I like to
hear the Old Man’s words, from time to time,

And take care,
when I’m with him, not to spew.

It’s very nice when such a
great Gentleman,

Chats with the devil, in ways so human,
too!