Since his feastday falls today and we have recently celebrated Trinity Sunday, it is appropriate to hear what he had to say on trinitarian doctrine. The excerpt following is from Alister E McGrath (ed), The Christian Theology Reader, 2nd ed, Blackwell 2001, pp 174-175.
This is the rule of our faith, the foundation of the building, and what gives support to our behavior.
God the Father uncreated, who is uncontained, invisible, one God, creator of the universe; this is the first article of our faith. And the second is:
The Word of God, the Son of God, our Lord Jesus Christ, who appeared to the prophets according to their way of prophesying, and according to the dispensation of the Father. Through him all things were created. Furthermore, in the fullness of time, in order to gather all things to himself, he became a human being amongst human beings, capable of being seen and touched, to destroy death, bring life, and restore fellowship between God and humanity. And the third article is:
The Holy Spirit, through whom the prophets prophesied, and our forebears learned of God and the righteous were led in the paths of justice, and who, in the fullness of time, was poured out in a new way on our human nature in order to renew humanity throughout the entire world in the sight of God.