This article is devoted to the cultural heritage of K. L. Kustodiev, a Russian writer and historian of the mid-nineteenth century, who served as a priest at the Russian diplomatic mission in Madrid in 1862—1870. Over the years of his life in Madrid K. L. Kustodiev has managed to collect a lot of information on the history and culture of Spain. The article analyzes Kustodiev’s ideas of Muslim Spain, the situation of Christian population and the church. The author comes to the conclusion that K. L. Kustodiev can rightfully be considered the first Russian historian of Christians in Muslim Spain.