system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] In the first movie, when Voldemort is acting through professor Quirrell, wouldn’t Voldemort know that Snape isn’t on his side when Snape was countering Quirrell’s magic to save Harry during the quidditch game?
gpt
Snape explains this in the Half-Blood Prince book.

He tells Voldemort that he was not aware that Voldemort was working through Quirrell, and only saw Quirrell himself trying to kill Harry and aquire the stone. This of course is a lie, because Snape knew Quirrell was possessed by Voldemort, but he needed to keep the illusion of loyalty to the dark lord to continue spying for the Order of the Phoenix.