The first dictionary definition of "cult" is "a system of religious worship." In other words, by the first definition, all religions everywhere are cults. This is different, however, from the way the word is most commonly used today.

The number of cult members worldwide changes radically based on which definition is used:

• A cult is any religious group that does not teach strict Christian doctrine. By this definition, more than 6.5 billion people are cult members, since even Catholicism is considered a cult by the most conservative Christian groups.
• A cult is a phenomenon supported with outspoken exuberance and devotion from its adherents. In this colloquial usage, actual membership is not the issue; the word "cult" is more a description of behavior or enthusiasm.
• A cult is a group that seems to take over people so that they lose the power to think for themselves. In this definition, cult membership is still impossible to determine because of disagreement about which groups qualify. The Chinese government considers Falun Gong a cult, but many people would not agree, including its 100 million practitioners. Some consider the Latter-day Saints (Mormons) a cult, though many do not—there are 12 million Mormons in the world. Many consider Wicca a cult, and many do not—there about 3 million practicing Wiccans. Many consider Scientology a cult—though the 500,000-plus Scientologists would disagree.

To make matters even more complicated, some religious groups choose not to collect or to share their membership numbers, for many reasons. Estimating cult membership remains impossible without a widely agreed-on meaning of the term and reliable figures for membership.