Complex sentence: Among other issues in social thought, the modernization of society and nationalism were two of his central themes, his multicultural perspective allowing him to work within the subject-matter of three separate civilizations--the Western, Islamic, and Russian.
Split into simpler sentences: Among other issues in social thought, the modernization of society was his "central preoccupation". His multicultural perspectivalism allowed him to work "within the subject-matter" of three separate civilizations--the Western, Islamic, and Russian.