Robert Jenson defines theology

The church has a mission: to see to the speaking of the gospel, whether to the world as message of salvation or to God as appeal and praise. Theology is the reflection internal to the church’s labor on this assignment. […]

Theology is thinking what to say to be saying the gospel.

Robert Jenson, Systematic Theology: The Triune God, Vol. 1. (OUP, 1997), 11, 32.

[The best definition of theology I’ve come across to date]