A historian of Western Medieval history by training and a religious philosopher by vocation, Lev Platonovich Karsavin (1882–1951) explored the interconnectedness of history and religion throughout his life. He developed his ideas focusing on the notions of theophany and theosis, where the divine revealed itself in history and history moved towards divinization. Karsavin’s writings reflected influences of such diverse thinkers as Nicolas of Cusa, Bonaventure and Angela of Foligno, or Vladimir Soloviev and Oswald Spengler. Building upon their ideas, Karsavin created an original philosophy of personalism, which was based on the concept of ‘all-unity’. Central to it was the idea of the ‘person’ both as an individual and a collective entity. This chapter examines Karsavin’s life and thought, first in pre-revolutionary St. Petersburg, through the experiences during the Russian Revolution, during his exile in Berlin, Paris, and Kaunas, and finally to his incarceration in Stalin’s Gulag camp where he was sent after the end of the Second World War and where he perished. It pays close attention to a number of crucial points of Karsavin’s intellectual odyssey including the impact of the revolution on Karsavin’s thought, the development of his concept of ‘all-unity’, his historiosophy, his engagement with ecumenism as well as his involvement with Eurasianism. The chapter concludes with a discussion of the rediscovery of Karsavin’s philosophical legacy in post-Soviet Russia.