Ante-Nicene Fathers, Vol VIII: The Decretals.: Preface.

p. 613 The Epistles of Pope Callistus. 2723 The First Epistle. 2724 On the Fasts of the Four Seasons, and that no one should take up an accusation against a doctor (teacher). Callistus, archbishop of the Church Catholic in the city of Rome, to Benedictus, our brother and bishop, greeting in the Lord. By the love of the brotherhood we are bound, and by our apostolic rule we are constrained, to give answer to the inquiries of the brethren, according to what the Lord has given us, and to furnish them with the authority of the seal of the apostles. Callistus succeeded Zephyrinus in the bishopric of Rome, and discharged the duties of that office for five years. This is all the information which Eusebius (in his Chronicon and Hist. Eccl., vi. 21) gives us in regard to Callistus. Later writers make many other statements. [See note, p. 618.] The letters attributed to him form part of the False Decretals of the pseudo-Isidorus, mentioned in the notice of Zephyrinus. 613:2724