The GM is the arbiter of whether or not you can take the Forbidden Knowledge Talent. Keep in mind that possessing such expressly disallowed information effectively marks you for execution by The Inquisition, should they find out. In rare circumstances, you might be taught forbidden knowledge by your Patron, if they have the knowledge themselves and they place great trust in you. Some unlucky individuals encounter Forbidden Knowledge during particularly perilous parts of their lives, such as encountering a tainted artefact or extensive spying on a xenos foe. In either of these cases, or any other in which your GM agrees you are justified in taking this Talent, the expenditure of XP represents your efforts to comprehend and internalise such reprehensible thoughts.
Advances in a Forbidden specialisation come with special difficulties and risks. The Imperium knows that knowledge is power, but it also knows that some power is extremely perilous and must be contained to "protect" humanity. Such dangerous heresy is locked away, far from curious eyes, to be shared only with specific individuals in extraordinary circumstances.
Daring to learn such heretical and prohibited matters first means the character must have access to this information or someone to act as an instructor. "Secondly, there are likely to be spiritual consequences which the GM may assign, up to and including assigning Corruption to the character". Furthermore, displaying expertise in such areas may draw unwelcome attention from all manner of other characters, from Chaos worshippers to Inquisitors and everything in between.