In theological texts the doctrine of the Trinity is often dealt with as part of Theology Proper. Personally however I find it more logical to deal with it as part of Pneumatology. For it’s not until the Holy Spirit is explored more fully that the relationship between the three persons of the Trinity can be explored more fully. And indeed one of the more debated aspects of the doctrine of the Trinity – the filioque clause – is focussed on the Holy Spirit. So, I tend to deal with monotheism as part of Theology Proper, the doctrine of the Incarnation as part of Christology, and the doctrine of the Trinity as part of Pneumatology.