A cult is a collection of people who believe in something that is usually not considered to be normal practice.
A small cult was formed on Bajor that worshiped the Pah-wraiths, believing them to be the true gods of Bajor. The Pah-wraith cult was considered a joke until late 2374, when Dukat released a Pah-wraith into the Bajoran wormhole, collapsing it, and starting a celestial war. A young member of the cult tried to assassinate Captain Benjamin Sisko on Earth several months later. (DS9: "Image in the Sand", "Covenant")