Gregg Allison identifies eight reasons for studying historical theology in his new book, Historical Theology: An Introduction to Christian Doctrine (Zondervan, 2011) which I reviewed recently.
Here they are (from pages 24-29):
1. Historical theology helps the church distinguish orthodoxy from heresy.
2. Historical theology provides sound biblical interpretation and theological formulations.
3. Historical theology presents stellar examples of faith, love, courage, hope, obedience and mercy.
4. Historical theology protects against the individualism that is rampant today among Christians.
5. Historical theology not only helps the church understand the historical development of its beliefs, but enables it to express those beliefs in contemporary form.
6. Historical theology encourages the church to focus on essentials, that is, to major on those areas that have been emphasized repeatedly throughout the history of the church.
7. Historical theology gives the church hope by providing assurance that Jesus is fulfilling his promise to his people [to build his church].
8. Historical theology [enables us] to enjoy a sense of belonging to the church of the past.
In addition to these listed reasons, Allison also says, ‘Historical theology can guard Christians and churches from the penchant for the novel, the yearning for relevancy, and the tendency to follow strong leaders who are biblically and theological shallow.’ (26)