St. Basil:
“Of the dogmas and messages preserved in the Church, some we possess from written teaching and others we receive from the tradition of the Apostles, handed on to us in mystery. In respect to piety, both are of the same force. No one will contradict any of these, no one, at any rate, who is even moderately versed in matters ecclesiastical. Indeed, were we to try to reject unwritten customs as having no great authority, we would unwittingly injure the gospel in its vitals; or rather, we would reduce [Christian] message to a mere term” (The Holy Spirit 27:66 [A.D. 375]).
St. John Chrysostom:
“[Paul commands,] ‘Therefore, brethren, stand fast and hold the traditions which you have been taught, whether by word or by our letter’ [2 Thess. 2:15]. From this it is clear that they did not hand down everything by letter, but there is much also that was not written. Like that which was written, the unwritten too is worthy of belief. So let us regard the tradition of the Church also as worthy of belief. Is it a tradition? Seek no further.” (Homilies on Second Thessalonians [A.D. 402]).
Vincent of Lerins:
“But it will be said, If the words, the sentiments, the promises of Scripture, are appealed to by the Devil and his disciples, of whom some are false apostles, some false prophets and false teachers, and all without exception heretics, what are Catholics and the sons of Mother Church to do? How are they to distinguish truth from falsehood in the sacred Scriptures? They must be very careful to pursue that course which, in the beginning of this Commonitory, we said that holy and learned men had commended to us, that is to say, they must interpret the sacred Canon according to the traditions of the Universal Church and in keeping with the rules of Catholic doctrine, in which Catholic and Universal Church, moreover, they must follow universality, antiquity, consent.” (Commonitory of the Antiquity and Universality of the Catholic Faith, 70 [A.D. 434]).