He identified the radical search for truth and the right for freedom of conscience as Servetus' main legacies, rather than his theology and scientific discovery.
He studied the influence of Servetus in the world and the development and ideas of the Socinian movement in the 16th and 17th centuries, precursors of the Enlightenment and modern era.
Marian Hillar has published studies on the philosophy of Hippocratic medicine; Greek philosophy of science and the origin of science; ancient Greek philosophy; liberation theology; the New Testament, Dead Sea Scrolls, early Christian writers, and the origin of Christianity; Philo of Alexandria, Numenius, early church fathers and development of the theory of logos and of the Trinity; studies in ethics- Stoics, Thomas Aquinas, Kant; Radical Reformation and development of antitrinitarian doctrines, Socinians; development of modern ideas on freedom of conscience and church-state separation.
Hillar's major theological work is demonstrating how the doctrine of the Trinity was developed from various sources through the evolution of the Jewish messianic expectations which underwent changes during centuries.
Hillar is founder and was editor-in-chief (1992-2012) of the Essays in the Philosophy of Humanism,[5] a scholarly journal published yearly since 1992 by the Humanists of Houston.