Socinianism is a Nontrinitarian Christian belief system developed and co-founded during the Protestant Reformation by the Italian Renaissance humanists and theologians Lelio Sozzini and Fausto Sozzini, uncle and nephew, respectively.

Nontrinitarianism is a form of Christianity that rejects the mainstream Christian theology of the Trinity—the belief that God is three distinct hypostases or persons who are coeternal, coequal, and indivisibly united in one being, or essence. Certain religious groups that emerged during the Protestant Reformation have historically been known as antitrinitarian.