He was expelled (1662) from Oxford for his religious nonconformity and was then sent by his father to the Continent to overcome his leanings toward Puritanism. He continued his religious studies, however, and in Ireland, where he had been sent (1666) to oversee the family estates, he became a staunch member of the Society of Friends. He was imprisoned (1668) for writing a tract ( The Sandy Foundation Shaken ) against the doctrine of the Trinity, but, undaunted, he wrote No Cross, No Crown and Innocency with Her Open Face while in the Tower of London. After his release (1669), Penn continued his writing, his many tracts including The Great Case of Liberty of Conscience (1670), in which he argued for religious toleration. He also went on preaching missions through England, the Netherlands, and Germany.