The Christian doctrine of the Trinity is the central doctrine concerning the nature of God in most Christian churches, which defines one God existing in three coequal, coeternal, consubstantial divine persons: God the Father, God the Son and God the Holy Spirit, three distinct persons (hypostases) sharing one essence/substance/nature (homoousion). As the Fourth Lateran Council declared, it is the Father who begets, the Son who is begotten, and the Holy Spirit who proceeds. In this context, one essence/nature defines what God is, while the three persons define who God is. This expresses at once their distinction and their indissoluble unity. Thus, the entire process of creation and grace is viewed as a single shared action of the three divine persons, in which each person manifests the attributes unique to them in the Trinity, thereby proving that everything comes "from the Father," "through the Son," and "in the Holy Spirit."

Christian theology is the theology – the systematic study of the divine and religion – of Christianity and Christian belief and practice. It concentrates primarily upon the texts of the Old Testament and of the New Testament, as well as on Christian tradition. Christian theologians use biblical exegesis, rational analysis and argument. Theologians may undertake the study of Christian theology for a variety of reasons, such as in order to:help them better understand Christian tenets make comparisons between Christianity and other traditions defend Christianity against objections and criticism facilitate reforms in the Christian church assist in the propagation of Christianity draw on the resources of the Christian tradition to address some present situation or perceived need education in Christian philosophy, especially in Neoplatonic philosophy