And you should know this day... that "Hashem is God, in the heavens above and on the earth below, there is none other (Deuteronomy 4:39).
One Torah commentary explains this passage to mean that if one has an awareness of God, there is nothing more to know. This idea requires clarification.
The principles of faith of Judaism that were revealed at Sinai and through the prophets are absolutes, and, as axioms, are not subject to argument. In this sense, it is proper to state that "about God," there is nothing more to know.
Judaism does not require stagnation of the mind, however. Within the framework of the basic principles, Judaism has always encouraged the persistent search for truth. Throughout Jewish history, great scholars - the Ari in Kabbalah, the Baal Shem Tov in Chassidus, Rabbi Yisroel of Salant in Mussar, and Rabbi Chaim Soloveitchik in Talmudic analysis, to name several more-recent leaders - have electrified their contemporaries and future generations with their original insights.
Judaism is vibrant, and the Jewish mind must be productive. Just as we cannot have a valid geometry if we postulate that two parallel lines intersect, so we cannot develop valid ideas of Judaism by abrogating any of the basic principles of the faith. The search for an ever-deeper understanding of Torah, however, should never end.