Mike Bird ends his articulation and apology for the structure of his systematic theology (Evangelical Theology) by summarizing the reason why "evangelical theology" should be ordered by the gospel itself:
In the end, evangelical theology is . . . a theology of the gospel. The gospel comprises the beginning point, boundary, and unifying theme for all theology.—Mike Bird, Evangelical Theology, 45.
It is also the interpretive grid through which our reading of Scripture takes place. The first 'word' in theology should be the 'word of the gospel' (Acts 15:7 RSV).
Doctrine is that which springs from the word of the gospel and provides the basis for the core teachings of the faith shared by all major Christian groups. Obviously an evangelical theology is one that lunges, leaps, works, worships, prays, and preaches from the gospel itself.
Where a theology cannot trace its trajectory back to the gospel, there it is not evangelical. The gospel is the rule of faith for the evangelical churches as it provides the lens through which we understand the mission of the Triune God and his work for us in salvation.