This is an uncommon use of "people." Here it isn't the plural of "person", but rather a collective singular noun meaning a nationality or a race.

Person = believes People = believe Examples: This person believes in a higher power. These people believe in a higher power. He walks. They walk. The chair breaks. The chairs break. When the noun is singular (person) the verb is “plural”. When the noun is plural (people), the verb is “singular”. The passage uses “A people”, which is very uncommon. But same concept applies 🤗 🌈 I hope that helped Nicky

Beware of the word "people" which can be either a singular collective noun (like "flock" or "crowd") or the plural of "person". If your sentence starts with "a", "people" can only be a collective singular noun and you need to say "A people that believes ..." On the other hand, if you want "people" to be the plural of "person" then you should say "People who believe ..."

I believe it has to do with plurality. So as an example you could say “Friends who believe in…” whereas you would say “A friend who believes in…” So if I understand it properly I think it simply has to do with “people” being kind of plural even though in this case they are using people almost as a singular. (As a note it is quite rare that people would say “a people” but in these cases it’s just being used almost synonymously with “group” to talk about a group of people who are usually connected by race, religion, or nationality. So you might hear “They are a very proud people.”)