Newman's first, and most deeply researched book, The Arians of the Fourth Century
is a major achievement in both social and theological history. As social history, it gives a detailed account of the moral and ideological temperament of the Arians in the Nicene era and the historical circumstances that caused their heresy to become unusually oppressive and politically powerful.
As a major contribution to theological history, it was in the course of composing The Arians that Newman first conceived his theory of the development of Christian Doctrine, which stimulated his conversion to Catholicism. This emerges from the book's in-depth study of Trinitarian doctrine in the Early Church and in the councils of the fourth century.
Most important, however, is Newman's deeply influential, concluding essay On Consulting the Faithful, which takes up the apparent problem that the Church proper seems to have fallen into heresy during the Arian ascendancy. In answer, Newman provides historical evidence that the faithful of Christ maintained orthodox doctrine even in the Church's darkest hour. He argues for the indefectibility of the Church and a nuanced theology of the sensus fidelium that impacted the Second Vatican Council's ecclesiology.