Roman Catechism 18.75

Chapter 18 — Part II: The Sacraments — The Holy Eucharist

Section 75. Testimony Of The Fathers — The first is by consulting the Fathers who flourished in the early ages of the Church and in each succeeding century, who are the most unexceptionable witnesses of her doctrine. All of these teach in the clearest terms and with the most entire unanimity the truth of this dogma. To adduce the individual testimony of each Father would prove an endless task. It is enough, therefore, that we cite, or rather point out a few, whose testimony will afford an easy criterion by which to judge of the rest.