Pope St. Dionysius of Rome

“Next I naturally turn to those who divide and cut into pieces and destroy that most sacred doctrine of the Church of God, the monarchy. . . . So we may not divide into three god- heads the wonderful and divine unity; nor disparage as a created being the dignity and exceeding majesty of the Lord; but we must believe in God the Father Almighty, and in Christ Jesus his Son, and in the Holy Ghost, and that the word is united to the God of the universe. For he says " I and the Father are I one" and " I am in the Father and the Father is in me ". In this way both the divine Trinity and the holy doctrine of the monarchy will be preserved.” (Adversus Sabellianos, preserved in Athanasius’s De Decretis Nicaenae Synodi [c. A.D. 260]).