It gets used that way but cults are not inherently religions there are plenty of non religious cults like nxivm or est until the 80s or the Manson family. Cults tend to have some relgiosity but the more important thing is to have some sort of figure that gathers people in a way that tends to be both extremely exploitative to then and often destructive to the people involved. Cults often coop a mainstream religion to be the justification for their behaviors even if most followers of the religion would not necessarily condone the actions of the cult.