How is it true that where knowledge should be strength, it turns out to be a weakness in Doctor Faustus.

This is an abstract question that doesn't have any real
evidential substantiation in the text of Marlowe's Doctor Faustus,
yet it is a central thematic point. In fact, it is the point upon which the whole
premise of Doctor Faustus is built.

In most
contexts, knowledge is recognized as the greatest liberating factor in life. If one
knows one's worth and rights, one will no longer submit to being oppressed by a tyrant's
or bully’s hand. If one knows the facts and abstractions and the fine points of thought
that come from an extensive education in academia or in a craft, one no longer is bound
to limited opportunity and poverty. If one knows the workings of the essential makeup of
the universe, one can liberate oneself from dependence upon manual labor; more
importantly, if one knows the dangers of experimental technology, one can protect
oneself, family and planet from toxic poisoning.

In Faustus's case, he
had mastered knowledge from medicine to law to theology and everything else in between.
The Chorus in Act I explains his academic mastery by saying that he was "glutted now
with learning's golden gifts," meaning he had mastered all available knowledge in every
available field of study. The Chorus also describes how the strength of knowledge has
turned to weakness for Faustus by comparing him in a metaphor to Icarus who, despite
warnings against it, flew too near the sun with waxen wings. They say of
Faustus:

Till
swoln with cunning, of a self-conceit,
His waxen wings did mount above his
reach,
And, melting, heavens conspir'd his overthrow;
For, falling
to a devilish exercise,
And glutted now6 with learning's golden
gifts,
He surfeits upon cursed necromancy;
Nothing so sweet as magic
is to him,

The strength of
knowledge turns to weakness when it becomes the all-consuming importance in life; when
one "prefers [knowledge] before his chiefest bliss," which, according to Marlowe, is
spiritual knowledge that must accompany other fields of knowledge; when one is willing
to forfeit one's inner "chiefest bliss" for all-consuming
greatness.