Nick has cited this Septuagint vs. Masoretic text "gotcha" to Protestants many times and
every time it's retarded. For context, the Septuagint is a Greek translation of the Hebrew Bible (aka Old Testament) that was produced in the 3rd-2nd centuries BCE. The Masoretic Text is a version of the Hebrew Bible in Hebrew; it's called the Masoretic Text because it was preserved by a group called the Masoretes in the 5th-10th centuries CE, and for a long time the oldest extant Hebrew Bible texts were from the 10th century CE. There is evidence that the authors of the New Testament quoted from both versions of the Hebrew Bible, and there are significant differences between the versions (for example, the books of Jeremiah and Job are shorter in the Septuagint). When Nick says that Protestants use the Masoretic Text, he is most likely referring to the King James Version, in which the OT was translated from the Masoretic Text (though the Septuagint was consulted for some verses with Christological significance, e.g. Psalm 22:16).
HOWEVER:
- Within the last century, many more ancient Hebrew texts of the OT have been discovered, most significantly the Dead Sea Scrolls dating to the 2nd-1st century BCE. Biblical scholar Emanuel Tov, one of the foremost experts on the Dead Sea Scrolls, categorized ~60% of the Biblical scrolls as "proto-Masoretic" (containing numerous, distinct agreements with the Masoretic Text) and ~%5 as "pre-Septuagint" (having distinct affinities with the Septuagint), and the other 35% aligned with neither. What's fascinating is, in one cave, three scroll fragments of Jeremiah were found, two aligning with the Masoretic text and one aligning with the shorter Septuagint but in Hebrew, indicating a high level of textual fluidity at the time. The Dead Sea Scrolls are a fascinating topic and there's too much to get into here, but the point is most modern Bible translations are based on critical editions of the Hebrew, Aramaic, and Greek texts and consult multiple sources.
- Though the KJV still has loyalists, there are 900+ English translations of the Bible, and the most popular English translation of the Bible is actually the New International Version. The NIV authors based their translation of the OT on the Masoretic Text, and additionally consulted the Dead Sea Scrolls, the Samaritan Pentateuch, the Greek Septuagint, Aquila, Symmachus and Theodotion, the Latin Vulgate, the Syriac Peshitta, the Aramaic Targums, and the Juxta Hebraica.
- The Old Testament in the Douay-Rheims Bible was translated from the Latin Vulgate. The official Latin version of the Bible promulgated by the Catholic Church today is called Novus Vulgatus, and it is also based on the Latin Vulgate. The Latin Vulgate is a Latin translation of the Bible produced by St. Jerome. Most of the Old Testament books in the Vulgate were translated directly from Hebrew (or Aramaic) by Jerome between 390-406 AD. The only exceptions are Psalms, which was a revision of an earlier translation from the Hexaplarian Septuagint, and the deuterocanonical books of Wisdom, Sirach, Baruch, 1 and 2 Maccabees, and additions to Esther and Daniel.
- The Catholic Church does not have a "Septuagint-only policy." This is obvious just looking at the textual basis of the Novus Vulgatus. In 2001, the Congregation for Divine Worship and the Discipline of the Sacraments (an administrative body of the Holy See) issued an instruction Liturgiam authenticam. This instruction establishes the Novus Vulgatus as "the point of reference as regards the delineation of the canonical text," and states that translations of liturgical texts "must be made directly from the original texts, namely the Latin, as regards the texts of ecclesiastical composition, or the Hebrew, Aramaic, or Greek, as the case may be, as regards the texts of Sacred Scripture[.]"
TL;DR Nick doesn't know what he's talking about.