The examination of the early layer in the Strategikon by Pseudo-Maurice and dating it to the 530s raises the question about correlation of the data on the Slavs contained in this early layer with the data of other sources about the Slavs of this period (first of all, Procopius and Pseudo-Caesarius). Their comparison shows almost complete consistency of the resulting image of the Slavs in the first third of the 6th century: Slavic society and culture appear as completely archaic and patriarchal, egalitarian and based on a lycanthropic cult. At the same time, sources on the Slavs of the last third of the same century (the indicated passages of Pseudo-Maurice being excluded) suggest a Slavic society with a developed institution of leaders' power. This allows us to make an assumption about a revolutionary development of the Slavs in the 6th century.