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The Septuagint is a Greek translation of the Hebrew Scriptures. It was written in Alexandria by, it is said, seventy scribes in seventy days, and was originally associated with communities of Hellenized Jews. Textual analysis of the New Testament reveals that Jesus and the early Church used the Septuagint, and it is the official Old Testament of the Eastern Orthodox Church. The Roman Catholic Church uses the Vulgate, a Latin translation.
At the Council of Jamnia in 90 CE many leading rabbis (partly to differentiate Judaism from Christianity) rejected the Alexandrian text in favor of the Hebrew texts (now known as the Masoretic Text); since seven of the books were available at the time only in Greek, this resulted in an Old Testament with only thirty-nine books rather than forty-six. In the Sixteenth Century the Reformation embraced this 7% smaller Bible (partly to differentiate Protestantism from Catholicism) and to this day Catholics have to answer charges that the Pope added books to the Bible. Nevertheless, the Septuagint is agreed by most Bible scholars to represent an older textual tradition closely related to the texts found at Qumran, and represents a valuable historical document of the evolution of the Bible.
A few King James onlyists consider the Septuagint a hoax, part of a Satanic conspiracy theory that claims that the early Roman Catholic Church intentionally sought to corrupt the Bible. Outside extreme fundamentalist circles, no one else takes this claim seriously.
YHWH versus "Lord"
By the time the Septuagint was written, the Hebrew name of God, YHWH, had become a "forbidden word" that a good Jew wasn't supposed to utter out loud. So, instead of transliterating YHWH into its Greek equivalent, some versions of the Septuagint uses the Greek word kyrios (lit. "Lord") every place the Hebrew scriptures contain YHWH. Older versions of the Septuagint apparently include YHWH. 
This explains why YHWH appears nowhere in the New Testament. The authors of the New Testament were using the Septuagint as their source of scripture. Every place an Old Testament passage is quoted in the New Testament, it is the Septuagint version of that passage that is quoted; e.g., while Isaiah 40:3 said "prepare the way for YHWH", Matthew 3:3 quoted it as "prepare the way for the Lord".
This tradition has carried forward into modern translations of the Bible, such as the King James version, in which the Hebrew YHWH is translated as "the LORD" in small caps.