In 1867 Cassel was appointed missionary by the "London Society for Promoting Christianity Amongst the Jews" (a Jewish Christian missionary society now known as the Church's Ministry Among Jewish People or CMJ), a position which he retained till March 1891. At the same time (1867) Cassel was assigned to the pastorate of the Christuskirche in Berlin, with over a thousand sittings, erected from 1863 to 1864 by the Society on Königgrätzer Straße (renamed as Stresemannstraße, now in Kreuzberg's part of the Friedrichstadt quarter). He remained in service there for twenty-four years.