I was quite surprised when I came across this passage in Justin’s Dialogue with Trypho:
And Trypho said, “Being shaken by so many Scriptures, I know not what to say about the Scripture which Isaiah writes, in which God says that He gives not His glory to another, speaking thus ‘I am the Lord God; this is my name; my glory will I not give to another, nor my virtues.’ ”
And I answered, “If you spoke these words, Trypho, and then kept silence in simplicity and with no ill intent, neither repeating what goes before nor adding what comes after, you must be forgiven; but if [you have done so] because you imagined that you could throw doubt on the passage, in order that I might say the Scriptures contradicted each other, you have erred. But I shall not venture to suppose or to say such a thing; and if a Scripture which appears to be of such a kind be brought forward, and if there be a pretext [for saying] that it is contrary [to some other], since I am entirely convinced that no Scripture contradicts another, I shall admit rather that I do not understand what is recorded, and shall strive to persuade those who imagine that the Scriptures are contradictory, to be rather of the same opinion as myself. With what intent, then, you have brought forward the difficulty, God knows. (Dialogue with Trypho 65, in ANF 1:230)
The Chicago Statement it ain’t, but it’s as close as you’ll get in the ancient world to a modern-day doctrine of inerrancy.