 Two years later he became dayyan and head of the yeshivah at Leghorn (Livorno), and in 1680 he returned to Amsterdam, where he was appointed head of the yeshivah 'Eitz Hayyim.After the death of Isaac Aboab da Fonseca (1693) he was appointed rabbi of the Portuguese community, which office he held till his death at Amsterdam.Jacob was one of the most violent antagonists of the Shabbethaian movement; he wrote many letters to various communities in Europe, Asia, and Africa, exhorting them to unmask the impostors and to warn the people against them.