Gian Paolo Alciati (c. 1515 – c. 1573) was an Italian antitrinitarian theologian and physician, a notable figure in the radical Reformation of the 16th century. Born in Piedmont, Italy, Alciati became an early proponent of non-Trinitarian views, which challenged the orthodox Christian doctrine of the Trinity.
Due to his heterodox beliefs, Alciati faced persecution in Catholic Italy and, like many other Italian evangelicals and radicals, was compelled to seek exile. He spent time in Switzerland, particularly Geneva, where he interacted with other Italian exiles and intellectuals, including Lelio Sozzini, a key figure in the nascent Unitarian movement.
Alciati subsequently moved to Poland, a region that at the time offered a notable degree of religious tolerance and became a significant hub for various Protestant denominations, including antitrinitarian groups. He actively engaged in the theological debates of the era, advocating for his Unitarian interpretations. His influence was particularly profound in Transylvania (present-day Romania), where he served in the court of John Sigismund Zápolya, the Prince of Transylvania, who was himself receptive to Unitarian ideas.
In Transylvania, Alciati collaborated with Giorgio Biandrata, another Italian physician and antitrinitarian, in promoting Unitarian theology. Their joint efforts contributed significantly to the establishment and growth of the Unitarian Church in Transylvania, which officially recognized Unitarianism as one of the accepted religions in the principality.
Alciati's life illustrates the intellectual and geographical diaspora of radical reformers during the 16th century. His work, alongside that of his contemporaries, played a crucial role in shaping the early development of Unitarianism, especially in Eastern Europe, thereby influencing subsequent generations of non-Trinitarian thinkers. The precise date and circumstances of his death are not definitively known, but he is believed to have died around 1573.