The same deep religious feelings that moved men to become heretics played a decisive role in the development of the two monastic orders that contributed most to the fight against heresies and strengthening of the organization of the Western Church. Life and work of the two founders of mendicant orders, St. Francis and St. Dominic, are crucial for the understanding of spirituality, church life and social development of the western world of the 13th century. It is important to lighten up their characters and achievement from the scientific standpoint.