In the front of the Rahlfs-Hanhart edition of the Septuagint there is a brief “History of the Septuagint Text” written by Rahlfs.
As Rahlfs describes the way first century Christians regarded the Old Testament, he casually drops a Greek word that occurs only once in the LXX and once in the NT. Of course, I had to look it up.

For in the first days of Christianity the Old Testament was the Holy Scripture κατ εξοχην for the Christians, just as it was for the Jews, and the Christian missionaries were able to discover a ready point of contact wherever there had already spread a knowledge of the Old Testament.