Marcin Ruar (German Martin Ruar, Latin Ruarius), pseudonym Aretius Crispinus) (1589–1657) was a Socinian writer, advocate of religious tolerance, and rector of the Racovian Academy from 1620 to 1622. Ruar was born in Krempe, Holstein, the son of a Lutheran preacher. He studied in Altdorf, where he became acquainted with and adopted the non-trinitarian of Fausto Sozzini, and then studied law at Leiden. He was a polyglot making frequent travels through Denmark, France, Italy, the Netherlands and the countries of Germany, promoting non-trinitarian teaching and pleas for tolerance, publishing and earning a reputation as one of the most educated men of his era. He was offered a professorship at Cambridge, but rejected the proposal, due to the requirement to acknowledge the Trinity. He moved to