St Gregory the Theologian
Archbishop of Constantinople and one of the Three Holy Hierarchs, St Gregory the Theologian is one of only three Fathers the Church honours with the name 'the Theologian'. His Five Theological Orations, preached at Constantinople in 380 against the Eunomians and the Macedonians, are the classic exposition of the doctrine of the Holy Trinity: the incomprehensibility of God, the eternal generation of the Son, and the divinity of the Holy Spirit, who is glorified together with the Father and the Son.