The Septuagint, abbreviation LXX, is the earliest extant Greek translation of the Old Testament from the original Hebrew. The Septuagint was presumably made for the Jewish community in Egypt when Greek was the common language throughout the region. Analysis of the language has established that the Torah, or Pentateuch (the first five books of the Old Testament), was translated near the middle of the 3rd century BCE and that the rest of the Old Testament was translated in the 2nd century BCE before Christ. The Septuagint is the only complete and reliable translation for the Old testament that was in Greek which is the most widely used language by the time of the early church.
In addition to all the books of the Hebrew canon, the Septuagint includes the deuterocanonical books, which are not included in the Masoretic text adopted by Jews. The Protestant churches use only the Masoretic text as the canonical Old Testament.
So why did the early church, and currently orthodox churches, use the Septuagint?
FIrst, our Lord and Savior Jesus Himself used the Septuagint as evidenced in many instances (click here for more details on this topic). For example, He confirmed the book of Maccabees as scripture by celebrating The Hanukkah. According to the gospel of Saint John: “Now it was the Feast of Dedication in Jerusalem, and it was winter.” (John 10:22, NKJV). The story of Hanukkah is preserved in the books of the First and Second Maccabees, which describe in detail the rededication of the Temple in Jerusalem and the lighting of the menorah. Those books are part of the Septuagint, but not canonized in the Hebrew Masoretic Text.
In addition, early Christians used the Septuagint out of necessity, since the language of most early Christians was Greek and they could not read Hebrew. Therefore, the Septuagint helped in the spread of Christianity in the early Church.
The Jews themselves used the Septuagint up to the second century CE. They stopped using the Septuagint altogether only in the second century after Christ, as many early Christians relied on the Septuagint to refer to prophecies that they claimed to have been fulfilled by Christ. The Jews considered this a misuse of Holy Scriptures and adopted the Masoretic text instead. The discoveries of the dead sea scrolls revealed copies of both Hebrew (pre-Masoretic) and Septuagint versions used by the communities of Qumran which assures that both Old Testament versions were used within the Jewish sects and that the Septuagint scriptures was regarded as divine scriptures by Jews.
The Greek text, not the original Hebrew, was the main basis for the Old Latin, Coptic, Ethiopic, Armenian, Georgian, Slavonic, and part of the Arabic translations of the Old Testament and has never ceased to be the standard version of the Old Testament in the Greek church. Indeed, St. Jerome used the Septuagint to begin his translation of the Vulgate Old Testament in 382 CE.
Finally, the Church’s theology was also explicitly shaped by the Septuagint. For example, St. Matthew’s reference to the prophecy of Jesus’ virgin birth is supported only in the Greek text (whereas the Hebrew has “young woman”). The writings of the Apostolic fathers are similarly saturated with quotations from the Septuagint, and specific Greek readings were used to refute heresies throughout Church history.