I was wondering if this question is grammatically correct. " How different is people's belief about magic today from that of people in the past?"

Except in rare cases, people, being plural and not indicating any kind of commonality, possess plural things. So the question is better phrased

How different are people's beliefs about magic today from those of people in the past?

That is still somewhat awkward, though, so for style, I would change it to

How have people's beliefs about magic changed from 20 years ago to today?

If the question is only about whether people believe in magic or not, then you can use the singular

How have people's belief in magic changed over time?

Since humans are the only sentient beings (on earth), one can permissibly dismiss the word 'people' all together and pose the question:

How has the belief in magic changed over time?

or

“ How different is the belief about magic today from that in the past?”