The Jewish Rabbi was not saying you don't know the words because you are not a Jew. There are tons of words in the Bible the best, wisest Jewish scholar in the whole world (dare I even say universe?) doesn't know, and that is what that Rabbi was telling you.
However the Jewish sages wrote (in the Oral Torah, without which no one can understand what is in the Torah, Prophets and Writings at all, not to mention all the later inspired commentaries such as Rashi) "The work is great (meaning very large), and one is unable to complete it, but neither is one allowed to turn away from it and not attempt it."
It is true that we only incompletely can follow God's law as laid out in the Bible and the numerous books of Oral Law and later case law, because even if we completely understood every word, we still, being finite human beings would but incompletely understand and perform God's will. Jews meditate and repent every year at Rosh Hashana and Yom Kippur, because we know that even the most pious and perfected Rabbi (pick any martyred Rabbi that you care to recall), can't fulfill all of God's will and the Law perfectly, and even such a saintly individual must repent of all the errors he or she made both to God and to other human beings.