Fragmentary Reading Notes and Autobiographical Reflections
p11“shortly before his death, his Concordia, wherein he proceeds upon the idea of the unity of church and state… Nevertheless there is great talent in his Concordia”
Fuller, Margaret. "Fragmentary Reading Notes and Autobiographical Reflections." Houghton Library, Harvard University.
Inferred Fuller explicitly names 'his Concordia' as a late work by Friedrich Schlegel dealing with unity of church and state. Concordia was a journal Schlegel founded in 1820 in which he expounded conservative Catholic views on political order. The passage with commentary on Schlegel's talent appears to come from notes Fuller took from a secondary source