Systematic theology is a fallible discipline; only the Bible is an infallible guide for faith and practice. However, theology should not be done in a vacuum – just as we can see farther spatially if we stand on the shoulders of giants, we likewise can see further theologically if we stand on the shoulders of the church fathers. One ignores the works of these great teachers at his own peril. As with other disciplines, he who ignores the past is condemned to repeat its errors.
Considering seriously the enduring teachings of the orthodox Fathers of the past is essential in constructing a viable evangelical systematic theology for the present. The church has struggled long and hard with understanding God’s revelations to us and, as a result, the historic orthodox expressions of Christian truth have stood the test of time. To summarize, an adequate evangelical theology must be molded in the context of the ecumenical truth of the historic orthodox Christian church.
While not everything that every orthodox Father said on every theological topic is binding on contemporary evangelical theology, nonetheless, no one has any right to claim orthodoxy for any teaching that has been condemned by any of the ecumenical creeds, confessions, or councils of the church of the first five centuries. Likewise, any teaching not addressed in the ecumenical creeds and councils that is contrary to the universal consent of the Fathers should be considered highly suspect. The burden of proof rests on anyone who wishes to hold to any such precepts; he must have overwhelmingly clear and convincing evidence from infallible Scripture.
These tests for orthodoxy can be summarized as follows: (1) What is contrary to ecumenical creeds, councils, and confessions is certainly unorthodox; (2) What is not addressed in the ecumenical creeds, councils, and confessions but is contrary to the universal consent of the Fathers is almost certainly unorthodox; and (3) What is contrary to the general consent of the Fathers is highly suspect. It is within these parameters that we employ the use of the teachings of the great theologians of the historic Christian church.
From: Systematic Theology (in One Volume) by Norman L. Geisler (Minneapolis: Bethany House, 2011), pp. 163-164.