I assume that you are asking about the iconoclasts from the Byzantine Empire. These are the best-known iconoclasts in Western History.
The iconoclasts were people who believed in the destruction of all religious images. They were not opposed to religion. Instead, they felt that, for religious reasons, it was wrong to make and display religious images. They felt this largely because of the fact that the 10 Commandments forbid the worship of "graven images." The iconoclasts felt that their church was accepting the worship of the icons and, therefore, they wanted to do away with those images.