From the 1590s, families whose ancestors had been forcibly baptised in Spain and Portugal began arriving in Amsterdam and openly returning to Judaism, which was possible there and almost nowhere else in western Europe. They called themselves the Portuguese Nation, spoke and printed in Portuguese and Spanish, and had to learn a religion many of them knew only from secrecy and rumour, which made the community both intensely observant and anxious about deviation. Its synagogue of 1675 is still lit by candles in brass chandeliers and still has sand on the floor. The community's presses supplied Hebrew books across Europe, and Menasseh ben Israel petitioned Cromwell to readmit Jews to England.
Discipline was maintained by the herem, a ban excluding a person from the community for a period. Uriel da Costa was banned twice for denying the immortality of the soul and the authority of rabbinic tradition, and killed himself after a public humiliation.
In July 1656 the same instrument was used against a twenty three year old member named Baruch Spinoza, in terms far harsher than usual, cursing him by day and night and forbidding anyone to speak with him or read anything he wrote. No reason was recorded. He never sought readmission, made lenses for a living and wrote the Theological-Political Treatise, which reads scripture as a historical document, denies that prophecy conveys philosophical truth, and argues for freedom of thought as a condition of a stable state. The ban has never been lifted.