Professor Siebert is mainly interested in the critical theory of subject, society, culture, and history of the Institute for Social Research at the Johann Wolfgang von Goethe Universität, Frankfurt a.M., Germany, or of the so called "Frankfurt School." He has worked particularly in the critical theory of religion as an integral part of the critical theory of modern society. He has applied the critical theory to different paradigms of Judaism and Christianity, as well as to Greek and Roman religions. He has explored especially the political-theological tendencies in Greek religion, Roman religion, Judaism, and Christianity.

At this time Dr. Siebert is involved in a major research work concerned with the critical theory of religion of Max Horkheimer, Walter Benjamin, Theodor W. Adorno, Erich Fromm, Herbert Marcuse, Alfred Sohn-Rethel, Jürgen Habermas, etc. It will be published under the title The Critical Theory of Religion: The Rescue of the Hopeless - the Longing for the Totally Other.