Fausto Paolo Sozzini, or simply Fausto Sozzini (Latin: Faustus Socinus (; 5 December 1539 – 4 March 1604), was an Italian Renaissance humanist and theologian, and, alongside his uncle Lelio Sozzini, founder of the Nontrinitarian Christian belief system known as Socinianism. His doctrine was developed among the Polish Brethren in the Polish Reformed Church between the 16th and 17th centuries, and embraced by the Unitarian Church of Transylvania during the same period. Fausto Sozzini recollected most of his uncle Lelio's religious writings by traveling over again his routes throughout early modern Europe, and systematized his Antitrinitarian beliefs into a coherent theological doctrine. His polemical treatise De sacrae Scripturae auctoritate (written in the years 1580s and published in England in 1732, with the title A demonstration of the truth of the Christian religion, from the Latin of Socinius) was highly influential on Remonstrant thinkers such as Simon Episcopius, who drew on Sozzini's arguments for viewing the sacred scriptures as historical texts. Life Sozzini was born in Siena, capital city of the Republic of Siena, at the time under the rule of the Holy Roman Empire. He was the only son of Alessandro Sozzini and Agnese Petrucci, daughter of Borghese Petrucci (born in 1490), and granddaughter of Pandolfo Petrucci. His father Alessandro Sozzini, the oldest of eleven brothers, was born in 1509 but died in 1541, in his thirty-second year. Fausto had no regular education, being brought up at home with his sister Fillide, and spent his youth in desultory reading at Borgo Scopeto, the family's country-seat. To the able women of his family he owed the strong moral impress which marked him through life; his early intellectual stimulus came from his uncle Celso Sozzini, a nominal Roman Catholic, but an esprit fort, founder of the short-lived Accademia del Sizienti (1554), of which young Fausto was a member. In 1556 his grandfather Mariano Sozzini the younger's will, left Fausto, as only son of the oldest son, one fourth of the family estates, which made him independent. Next year he entered the Accademia degli Intronati, the centre of intellectual life in Siena. He joined with the name Frastagliato, while Celso had the name Sonnacchioso. About this time the jurist Guido Panciroli describes him as a young man of fine talent, with promise of a legal career; but he showed little interest for law, preferring to write sonnets. In 1558–1559 the suspicion of Lutheranism