The teachings of Igumen Theodosius to Prince Izyaslav cover significant aspects of Christian doctrine and practice. He discusses the permissibility of meat consumption on Sundays, the heretical nature of Latin beliefs, and the divine punishments for sin. Theodosius emphasizes the importance of repentance, fasting, and the need for Christians to maintain their faith amidst challenges. He also encourages acts of mercy and love towards others, reminding the faithful of their commitment to God and the necessity of enduring trials with patience. Overall, the text serves as a spiritual guide for maintaining true Christian values.