Original sentence: He was a major personality in the Ukrainian national awakening, a proponent of slavophile ideology, popular in the progressive intelligentsia of his time.In 1840s he founded a secret political organization called the  Brotherhood of Saints Cyril and Methodius in Kiev (for which he suffered arrest and exile), and through the 1860s to the 1880s, he continued to promote the ideas of federalism and populism in Ukrainian and Russian historical thought
Edited sentence: As a historian, Kostomarov was a proponent of populism and federalism; that is, he strove to write the history of the commom people, especially the common Ukrainian people, and was very innovative in his use of folksong and ethnography to do this.He believed that folksong revealed the true spirit of a people and thought that both in their history and in their national songs, the Ukrainian and Russian peoples differed from each other, as well as from their close neighbours, the Poles.He thought that the Russians were more autocratic in nature, the Poles more aristocratic, and the Ukrainians more democratic.He was a major personality in the Ukrainian national awakening, the founder of the Pan-Slavic but democratic 1840s Brotherhood of Saints Cyril and Methodius in Kiev (for which he suffered arrest and exile), and through the 1860s to the 1880s, as mentioned above, a proponent of federalism and populism in Ukrainian and Russian historical thought.Kostomarov saw federal principles at work in the early history of Rus', which was a varied and highly decentralized entity before the emergence of the Tsardom of Muscovy in the seventeenth century