The ultimate end of theology is not bare knowledge, but action.
Gerhard, Theological Commonplaces, Ex 1, p. 21.
When Gerhard writes about the “end” of theology, he means its goal, what it’s driving toward. We are poor theologians until we act upon what we have learned of God. Our acts should express themselves toward God our Maker, from who we learned theology, and toward those around us, whom God loves.
Toward God, our action is prayer and praise. Toward mankind, it is likewise praise of our (and their) Maker that manifests itself in mercy. (Sometimes mercy manifests itself in rendering judgment upon those who do evil, so that others might be delivered from evil: e.g., military and police work.) In acting on our knowledge of God, we join ourselves to God’s end: to save and treasure His creation through His beloved Son.