1 What was transacted concerning ecclesiastical faith at the Great Council assembled at Nicæa, you have probably learned, Beloved, from other sources, rumour being wont to precede the accurate account of what is doing. But lest in such reports the circumstances of the case have been misrepresented, we have been obliged to transmit to you, first, the formula of faith presented by ourselves, and next, the second, which [the Fathers] put forth with some additions to our words. Our own paper, then, which was read in the presence of our most pious Emperor, and declared to be good and unexceptionable, ran thus:—
2. “As we have received from the Bishops who preceded us, and in our first catechisings, and when we received the Holy Laver, and as we have learned from the divine Scriptures, and as we believed and taught in the presbytery, and in the Episcopate itself, so believing also at the time present, we report to you our faith, and it is this:”—
3. “We believe in One God, the Father Almighty, the Maker of all things visible and invisible. And in One Lord Jesus Christ, the Word of God, God from God, Light from Light, Life from Life, Son Only-begotten, first-born of every creature, before all the ages, begotten from the Father, by Whom also all things were made; Who for our salvation was made flesh, and lived among men, and suffered, and rose again the third day, and ascended to the Father, and will come again in glory to judge the quick and dead. And we believe also in One Holy Ghost:”
believing each of these to be and to exist, the Father truly Father, and the Son truly Son, and the Holy Ghost truly Holy Ghost, as also our Lord, sending forth His disciples for the preaching, said, “Go teach all nations, baptizing them in the Name of the Father and of the Son, and of the Holy Ghost.” Concerning Whom we confidently affirm that so we hold, and so we think, and so we have held aforetime, and we maintain this faith unto the death, anathematizing every godless heresy. That this we have ever thought from our heart and soul, from the time we recollect ourselves, and now think and say in truth, before God Almighty and our Lord Jesus Christ do we witness, being able by proofs to show and to convince you, that, even in times past, such has been our belief and preaching.
Source: Letter on the Council of Nicaea (New Advent)