Why did Voldemort still trust Snape?

Question

I recently watched the first Harry Potter movie and noticed a pretty big plot hole.

As we know, Snape turns over once Lily Potter dies and runs to Dumbledore for help and becomes a spy ultimately. Yet through the remainder of Voldemort's life, he believes that Snape is on his side yet he should have found out in the first movie.

As we know, Quirrell for the entirety of the first had Voldermort on the back of his head, living and breathing off him which means, shouldn't he also hear everything too?

When Harry and Quirrell are with the Mirror of Erised, he tells Harry that Snape was trying to counter curse his broom during his Quidditch match. Quirrell also gets confronted by Snape in the hallway (when Harry was snooping in the restricted section) telling him, he needs to figure out where his loyalties lie.

My question is, why didn't Voldermort hear Snape talking to Quirrell, figure out he was on Dumbledore's side and kill him once he rose?

Answer 1

Snape is a master Occlumens — that means he can shield his thoughts from those with skill in Legilimency, which is basically mind reading. Both Dumbledore and Voldemort are experts in this particular skill.

When Voldemort does rise again, Dumbledore asks Snape to resume his spying role with the Death Eaters. Snape is able to do this because of his ability to hide his thoughts, and can explain away his actions as trying to stay on Dumbledore's good side while he waited for his master to return.

I can't remember if it was mentioned in any of the films, but there is a part in one of the books where Bellatrix Lestrange questions Snape's loyalty, and he replies that if only the Dark Lord had trusted him rather than just possessing Quirrell, he would have been reborn years earlier.

Answer 2

This is relevant: Why didn't Voldemort suspect Snape as a double agent during Philosopher's Stone?

Basically, Snape would have had no way of knowing that Voldemort was under the turban. He also didn't know that Quirrell was attempting to assist Voldemort, and directly tells Voldemort in book 6 that he was trying to stop Quirrell to maintain Dumbledore's trust. This would have given him an excellent position inside Hogwarts when Voldemort returned.