Theologically and not only theologically, the Revelation at Mount Sinai is the core of Judaism.
And this not only because it is the beginning but because it is apprehended as a total and all-inclusive revelation.
That is, this revelation is considered the opening point, the transition point, between the higher essence and the lower essence—between God and man.
After this revelation there is actually no need for a new revelation because besides being the first or original of its kind, the Revelation is a one-time event that includes all the other revelatory events.
It has been compared to the primordial act of the creation of the world, which was also a first and single act and included all that was and will be in the world.
So, too, the Revelation at Mount Sinai is such a unique event containing in it all that afterward will ever be made known about the connection between God and man.
Therefore the Jewish tradition is full and complete—not because it relies only on an ancient single source, the Bible, but because it is open to additions.
All the accumulated oral traditions are considered part of the original written Torah.
Even details of the oral Torah, obviously belonging to a much later period, are considered to be continuations of the original revelation.
It is all the same revelation, written or oral, and includes the ancient text and the ever-changing unwritten social form and custom.
–Rabbi Adin Steinsaltz
From After the Bright Light of Revelation: A Conversation with Rabbi Adin Steinsaltz in On Being Free by Rabbi Adin Steinsaltz