Lutheran – but really Reformed at heart – theologian Robert Jenson begins his systematic theology with a prolegomena, much different than those used to reading systematics would expect. Jenson feels no lead to offer a lengthy defense of theology as a discourse, science, or method. He simply states that theology is what the church does so long as it is compelled to proclaim and enact the gospel in particular times and places. Theology is what the church does when it thinks about the gospel: what the gospel demands us to say about God, humanity, and the world and how it compels us to live.
Theology is the discourse of the church about the gospel.
The purpose that constitutes and distinguishes the church and in service of which the church needs to think is maintenance of a particular message, called “the gospel.” It happened during the reign of Tiberius at Rome that certain Jews believe themselves to have encountered the profit and Rabbi they had followed, Jesus of Nazareth, alive after his execution, endured somehow “for their sake,” and to have encountered him so situated over against his own death as to preclude his dying again. Given Israel’s grasp of death and life, they could report such events only by saying, “the God of Israel has raised his servant Jesus from the dead.” And given Israel’s interpretation of God and the meaning that “raised” must have within her linguistic world, such a risen one must merely thereby be established as somehow Lord of all; the creeds’ direct progression from “rose” to “is seated at the right hand of God” to “will judge the living and the dead” traces a straightforward conceptual nexus. – p. 4
The church is the community of ongoing witness to the belief that Jesus of Nazareth – the crucified and risen one – is Lord of all. There is no external or prior epistemological justification for this claim. This is what the earliest apostles declared they had seen and understood and we carry their message forward in word and deed, and in so doing we are the church as we theologize.