The paper considers the motive of unbelief as a phenomenon bordering on betrayal based on the gospel story about the assurance of Thomas. The main object of the study is a sermon from the manuscript collection "Statir" of the XVII century. The purpose is to identify the author's position, the rhetoric used in this regard and the way the text is organized. For a more complete disclosure of the precedent plot in the preaching context of the XVII century, sermons on this topic from the collections “Spiritual Sword” by Lazar Baranovich (1666), “Spiritual Dinner” by Simeon Polotsky (1681) are attracted.