[T]he authority of the Septuagint is pre-eminent as far as the Old Testament is concerned; for it is reported through all the more learned churches that the seventy translators enjoyed so much of the presence and power of the Holy Spirit in their work of translation, that among that number of men there was but one voice. And if, as is reported, and as many not unworthy of confidence assert, they were separated during the work of translation, each man being in a cell by himself, and yet nothing was found in the manuscript of any one of them that was not found in the same words and in the same order of words in all the rest, who dares put anything in comparison with an authority like this, not to speak of preferring anything to it? (paragraph 22)
Whatever criticisms we may make of this argument for the basis of the authority of the LXX (and the story has long been regarded as spurious), at the very least we must concede that it was considered an authoritative translation of Scripture, as evidenced not least by the fact that the apostles quoted from it.
St. Augustine also argues that the LXX was providentially ordained for the preservation of canonical books the Jews later omitted:
Wherefore, even if anything is found in the original Hebrew in a different form from that in which these men have expressed it, I think we must give way to the dispensation of Providence which used these men to bring it about, that books which the Jewish race were unwilling, either from religious scruple or from jealousy, to make known to other nations, were, with the assistance of the power of King Ptolemy, made known so long beforehand to the nations which in the future were to believe in the Lord. [ibid.]
My purpose here is to show that St. Augustine certainly did not seem to consider the opinion of the Jews as definitive for the canon of the Old Testament, as we have seen in another post.
Some might try and jump on the fact that St. Augustine received as true a tradition concerning the LXX that we know to be false, in order to suggest that tradition cannot be trusted. But the Catholic doctrine of infallibility does not extend to questions of historicity of the Septuagint. The Catholic doctrine of infallibility relates to questions of faith and morals. The fact that St. Augustine held to a view of the LXX that we know to be false in no way undermines the validity of Sacred Tradition for the transmission of the Faith.