The word "cult" has fallen on hard times. Used authentically, it refers to a grouping of people for some religious purpose; it can also refer to specific ceremonial, liturgical, and prayer activities carried out within a particular group. Vatican II, for example, refers to the "cult of the saints," meaning the honor and devotion Christians show to Christians who are now reigning with Christ in heaven. Used this way, "cult" carries no pejorative connotations.
In the last few decades an unfortunate phenomenon has sprung up, primarily among Evangelical Protestants who have appropriated the word and used it to categorize religious groups with whom they disagree. Mormons and Jehovah's Witnesses have become "cultists," and their religions are branded as "cults." In popular jargon "cult" implies more than just a religion with odd tenets. It carries the implication that the group has a hidden agenda, uses deception and mind control techniques to keep its members in line, and may be satanic in origin. Calling someone a "cultist" has become a handy stick with which to beat members of minority religions. Some Fundamentalists call the Catholic Church a cult.
Of course, some religions are cults, but it's a matter of prudence whether to trumpet that fact. If you want to evangelize adherents to such religions, you must avoid approaches that will alienate them. Be firm but charitable. Don't throw around the terms "cult" and "cultist." With a little restraint you'll more likely get your message across. If you start by telling a non-Catholic that he's a member of a cult (even if he is), it's unlikely that he'll listen to anything you have to say.