Under what circumstances does “belief ” use the plural form “beliefs” ? Does “beliefs” have any unique sense distinguished from “belief” ?
As a noun, "belief" can refer to a single thing that one believes, but can also refer to a complicated aggregate of many beliefs. For instance, one may refer to "Christian Belief", representing not the individual things which Christians believe, but the system of ideas which Christianity is thought to entail, each of which is often supposed to accompany the others. (Compare with "Buddhist thought".)
"Beliefs" is used when you want to explicitly refer to the fact that there is more than one idea which is being believed in. For contrast with "Christian Belief", the phrase "Christian Beliefs" would take the notion of Christian ideology and suggest that it contains potentially separable pieces, each of which is an idea in its own right which might be believed separately from the others.