Most Post Nicene Fathers are in fact Postmillenial in their approach to Eschatology. Most notably would be Saints Eusebius & Athanasius. Their era tended to lend credibility to this position as the were "watching the idols of men fall down before the living Messiah" (some where in "On the Incarnation of the Word). There is no reason to believe that a David Chilton or a Ken Gentry would have to change there eschatological views to convert to the Holy Faith of the Apostles, Orthodoxy. We would have to retro actively anathematize already canonized Saints of our faith in order to consider Postmillenial thinking as heretical or outside of the realm of Orthodox thought.