He became a Jesuit in 1639; was assigned to the Ethiopian mission, and arrived at Goa by the land route, via Isfahan. He worked first on the Island of Salsette off Goa, where from time to time he acted as Portuguese interpreter. He was sent on an embassy to one of the native princes, and finally reached the empire of the Great Mogul, where, as rector of the residence at Agra, he was involved in the persecution under Shah Jahan.
Here the French explorer, Francis Bernier, learned to know and appreciate him as one versed in expert knowledge of the philosophy of religions in India. In 1662, Roth revisited Europe by the land route via Kabul to obtain new recruits for the mission, and returned to Agra in 1664. Roth shares with his fellow Jesuit, Johann Ernst Hanxleden, the fame of being among Europeans the pioneer Sanskrit scholar, and of having compiled the first Sanskrit grammar. During his stay in Agra, he succeeded in persuading some Brahmins to teach him Sanskrit and, after six years of study, he obtained complete mastery of it. He was the author of the description of the Sanskrit alphabet published by Athanasius Kircher in his "China illustrata.
Roth's works, most of which were published by his friend Kircher, are: