In 1631 he tried to return to Holland, but the authorities remained hostile to him. He moved to Hamburg in 1632. But as early as 1634, the Swedes - a European superpower - sent him to Paris as ambassador. He remained ten years in this position where he had the mission to negotiate for Sweden the end of the Thirty Years War. During this period, he had been interested in the unity of Christians and published many texts that will be grouped under the title of "Opera Omnia Theologica".