You always have to use beliefs as believes is a third party singular form of the verb believe. The reason to use beliefs is to prevent any confusion. There are some more examples:
- Nouns whose verbs look similar:
Belief - Beliefs, Proof - Proofs, Grief - Griefs
- Words with two f's:
Cuff - Cuffs, Sheriff - Sheriffs, Plaintiff - Plaintiffs, Whiff - Whiffs, Staff - Staffs
- Other words:
Brief - Briefs, Reef - Reefs, Chief - Chiefs, Fife - Fifes, Roof - Roofs, Handkerchief - Handkerchiefs
Compare them with knives, wolves, lives, selves, wives, halves, etc.