The Apology of Aristides of Athens is our earliest extant complete defense of the Christian faith (ca. 125). Lost for centuries, part of it was rediscovered in 1878 in Armenian and, later, the complete text was discovered in Syriac in the monastery of St. Catherine in the Sinai.
When he comes to tell his addressee, the Emperor Hadrian, how he may confirm the teachings advocated in the treatise, where does he instruct him to turn? To their “writings,” of course, because it was their writings that were to be used to establish doctrine.
Indeed, this is how Aristides came to his knowledge of the Christian faith and its possession of the truth:
But the Christians, O King, while they went about and made search, have found the truth; and as we learned from their writings, they have come nearer to truth and genuine knowledge than the rest of the nations. (Apology 15)
He returns to this point again in the following paragraph:
And as for their words and their precepts, O King, and their glorying in their worship, and the hope of earning according to the work of each one of them their recompense which they look for in another world,-you may learn about these from their writings. It is enough for us to have shortly informed your Majesty concerning the conduct and the truth of the Christians. For great indeed, and wonderful is their doctrine to him who will search into it and reflect upon it. And verily, this is a new people, and there is something divine (lit: “a divine admixture”) in the midst of them.
Take, then, their writings, and read therein, and lo! you will find that I have not put forth these things on my own authority, nor spoken thus as their advocate; but since I read in their writings I was fully assured of these things as also of things which are to come. And for this reason I was constrained to declare the truth to such as care for it and seek the world to come. (Apology 16)
In this brief section, Aristides repeats three times that “their writings” are the source (the only one he mentions, in fact) for Christian dogma–the sole principium cognoscendi externum, to use the later terminology. He is careful to distinguish their authority from his own–for he does not claim to have any of his own. The authority of his words is simply the authority of truth, which Hadrian is exhorted to verify for himself. He does not even feel the need to be their “advocate”; in the last analysis, the writings need no advocate. On the contrary, they serve as his advocate or helper:1 they give Aristides “full assurance” of the “things which are to come.” Aristides only “declares.” He serves as witness to the Witness whence Christian teachings come with divine authority. Aristides’ words are an indication, pointing his readers back to the Law and to the Testimony, where we find the divine address and summons of the Triune God.