Newton never made a public declaration of his beliefs. ------------ And here is why. (1) By the 16th century, antitrinitarian movements were strong in Europe. For example, Ferenc Dávid (1510-79), a Hungarian, knew and taught that the dogma of the Trinity was not Scriptural. Because of his beliefs, he died in prison. (2) The Minor Reformed Church, which flourished in Poland for about a hundred years during the 16th and 17th centuries, also rejected the Trinity, and adherents of that church spread literature all over Europe, until the Jesuits succeeded in having them banished from Poland. (3) Sir Isaac Newton (1642-1727), in England, rejected the doctrine of the Trinity and wrote detailed historical and Scriptural reasons for doing so, but he did not have these published during his lifetime, evidently out of fear of the consequences.
