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.