A discussion about heresies and sects will help to see the history of Christianity as a field of sharp disputes, schisms, and searches for truth. Behind every schism stand people, ideas, fears, attempts to explain the structure of the world and find one's own path to truth. The lecture addresses those topics that often remain on the periphery of school and university courses, although without them it is difficult to understand the development of the Christian church, European culture, and public thought. Listeners will trace how the prerequisites for heresies arose, why alternative teachings appeared as early as the first centuries of Christianity, and how the church responded to the challenges of the 1st to 3rd centuries. Special attention will be given to the era of the Ecumenical Councils of the 4th to 8th centuries - a time when important theological decisions and boundaries of acceptable interpretations were formed. The conversation will then move on to the medieval movements of Western Europe, Russian sectarianism, the Reformation, and post-Reformation communities.