5 On their dictating this formula, we did not let it pass without inquiry in what sense they introduced “of the essence of the Father,” and “one in essence with the Father.” Accordingly questions and explanations took place, and the meaning of the words underwent the scrutiny of reason. And they professed, that the phrase “of the essence” was indicative of the Son's being indeed from the Father, yet without being as if a part of Him. And with this understanding we thought good to assent to the sense of such religious doctrine, teaching, as it did, that the Son was from the Father, not however a part of His essence. On this account we assented to the sense ourselves, without declining even the term “One in essence,” peace being the object which we set before us, and steadfastness in the orthodox view.
Source: Letter on the Council of Nicaea (New Advent)