It is debatable which is more reliable–the Septuagint or the Masoretic Text.  Let me put it this way: in making English translations of the Hebrew Old Testament, as a general rule, the opening text to be used in the Masoretic.  One obvious advantage of the Masoretic Text, even though it is not as old as the Septuagint, is that it is in Hebrew.  The Septuagint is a Greek translation of the Old Testament.  The translation was done in the late third or early second century BC, which lends credence to the Septuagint, because the Masoretic Text was produced in the eighth or ninth centuries AD.  However, because it is a translation, it is one layer removed from the original Hebrew.

Having said that, if you read your Old Testament, you will find that in a number of passages the translators prefer the Septuagint for various reasons.  If there is evidence of a problem of copying or interpolation in the Hebrew, the Septuagint, being nearly one thousand years earlier, is often referred to.  So, there is no simple answer, but it I had to choose the “more reliable” version, it would be the Masoretic.

By the way, there is a third candidate for the “most reliable” version of the Old Testament, which is the Dead Sea Scrolls (DSS).  These are a varied set of Hebrew manuscripts, primarily for the third to first centuries BC.  A limitation of the DSS, is that they are partial.  Large swathes of the Hebrew texts are not found in the DSS.  To make things even more complicated, sometimes the DSS are closer to the Septuagint, and sometimes they are closer to the Masoretic.  Let us put it this way: We are very fortunate to have three separate witnesses to the Hebrew Old Testament, which is the Septuagint, the DSS and the Masoretic Text.

Yes, it is true that in the very early church in Jerusalem, Judea and Samaria, the disciples did continue to hold to much of the Jewish traditional practices, such as Sabbath-observance, kosher laws and the festivals.  As Christianity spread to the Gentile world, Jewish practices were not abandoned by all Christians, but the percentage keeping to Sabbath, the dietary laws and so forth lowered considerably.  By the late first century, Jewish leaders began to anathematize Christians, which caused a hard turn away from Judaism and Jewish practice.  I would mildly correct your statement by saying that the de-Judaizing process happened principally in the late first century, and was nearly complete by the second century AD.  In fact, by the second century the church had already taken an unfortunate turn toward anti-Semitism, beginning a shameful chapter in the Christian church.

John Oakes