Kingdom of the Cults gives an overview of cults and other religions from the perspective of Christianity. Cults, as defined by the book, are groups of people who meet around a specific person or person's misinterpretation of the Bible. The cults described in the book, while very different in beliefs, do have some similarities. Many of the cults do not believe in Jesus' virgin birth, his incarnation as fully man and fully divine, his miracles, his death on the cross, his substitutionary sacrifice, his power to cleanse people from sin, or his bodily resurrection nor do many cults believe in the Trinity. All of these are foundational Christian doctrines.

Several cults deny the existence of man's fallen nature, hell, Satan, sin, evil, death, and the need for redemption. Some of the cult doctrines include unbiblical beliefs such as: God is all in all, man will be exalted to godhood, there is oneness in everything, God is an impersonal God, there is no need for prayer, and man can achieve salvation or enlightenment for himself. The founders of the various cults usually receive "a vision" and many go on to publish writings (which sometimes result in accusations of plagiarism). Some of the founders of the cults prove to be of corrupt character and have dealings with the law.

Many of the cults are very proactive in mission work to recruit more converts and have their writings published in a multitude of languages. The author gives suggestions for how Christians can share their faith with cult members with the first step being to read this book and educate themselves on the Word of God and the beliefs of those outside Christianity. The author stresses to Christians the importance of communicating love and sincerity to cult members.