Christianity takes its name from Christ. So one might naturally suppose that the heart of Christian doctrine should lie in Christology—as it seems to do in the New Testament. But Christology has a doctrinal background even in the New Testament, and, when it came to be formulated in creeds and symbols of faith, it was finally located in the context of the doctrine of the Trinity—the belief that God, though one, is also somehow three. For this reason one can say that the key Christian doctrine is that of the Trinity. At any rate, this is how Aquinas sees things. He thinks that Christian teaching is certainly about Christ. But he does not think that it is just about Christ or primarily about Christ. For him, the heart of Christian teaching is the doctrine of the Trinity, which is the first specifically Christian topic he turns to in the Summa theologiae. 'After discussing the unity of God's nature', he explains, 'it remains for us to discuss the trinity of persons in God.' 1 Historically speaking, Aquinas is one of the most important writers on the doctrine of the Trinity.
For a classical statement of the doctrine of the Trinity we could refer to the creed promulgated by the First General Council of Constantinople (381), which reaffirms the teaching of the Council of Nicaea (325), and which adds that the Holy Spirit is 'the Lord and Giver of life who proceeds from the Father, who together with the Father and the Son is worshipped and glorified'. 2 For a more systematic statement, however, we can cite the so-called Creed of Athanasius, a document produced some time after the middle of the fifth century