In a pamphlet published a short time before his death, he complained of the inconsiderate treatment he had received at the hands of his Christian friends ("Erstes Sendschreiben an Freunde in Deutschland und England über die Christuskirche in Berlin und ihr Martyrium durch die London Society", 1891). Hermann Leberecht Strack states that it is not clear what induced Cassel to join the Christian Church, though he contends that Cassel's reasons were obviously not mercenary. (See Herzog-Hauck, "Real-Encyc." iii. 744). Cassel combated anti-Semitism with considerable warmth (in "Wider Heinrich von Treitschke: Für die Juden", 1880; "Die Antisemiten und die evangelische Kirche: Sendschreiben an einen evangelischen Geistlichen", 1881; "Ahasverus: Die Sage vom ewigen Juden; eine wissenschaftliche Abhandlung; mit einem kritischen Protest wider Ed. v. Hartmann und Adolf Stöcker", 1885; also "Der Judengott und Richard Wagner: Eine Antwort an die Bayreuther Blätter; Zum 28. Mai 1881"). He wrote and spoke against Adolf Stöcker and Richard Wagner and other expressions of Protestant anti-Semitism during a period of its resurgence.