“And Hashem spoke to Moshe on Mt. Sinai saying: Speak to the children of Israel and say to them: When you come to the land that I give you, the land shall observe a Shabbos rest for Hashem” (Vayikra 25:1-2).
Rashi, 11th century French sage, quotes the comments of the Toras Kohanim (Mishnaic era 200-100 B.C.E) on this verse. The Toras Kohanim asks, why is it necessary for the Torah to state that Hashem spoke to Moshe on Mt. Sinai . Weren’t all the commandments given there? He goes on to explain that, just as all the details are brought down regarding this mitzva, so too in all the mitzvos, every detail was given over at Sinai.
If this true, however, then why does it not list all the details of every commandment given down? When Hashem gave us the Torah at Sinai 3300 tears ago, He in reality gave us two components of the Torah: the written part and the oral part. The written part is what we call the Chumash, the Bible, or the five books of Moshe (Prophets, such as Joshua, Isaiah, etc. and Writings, which include Psalms, Proverbs, etc. were written later as the events occurred).
The reason Hashem gave us the Torah in such a fashion, as opposed to writing details of every mitzva in the written law, is to keep the Torah alive amongst the Jewish people. Something that is written down could possibly become irrelevant at some point. However, the Torah has a written element that can only be understood through using the oral element.
After the Torah was given, generation after generation would read the written part, and using the oral Torah, they would explain all the details relevant to the verse. The Laws of Shabbos provide an example. The written Torah says to remember and safeguard the Shabbos. The oral Torah Shabbos explains exactly what it means to remember and safeguard the Shabbos. This method required constant study, which in turn created an intellectual vibrancy that led to true love of the Torah. In fact it states in the Talmud that in the time of King Chizkiah there was not a small child found in the land of Israel who did not know the laws of ritual purity.
Furthermore, it is impossible to truly comprehend even one commandment of the written Torah without the oral Torah.
Right before the destruction of the 2nd Temple the Rabbis realized the Jews might lose their chain of transmission in the exile. Therefore, they permitted the oral law to be written in an obscure form that on the one hand would ensure that it would never be forgotten, but on the other hand would still require diligence to understand it. This is the Talmud.
To assume the Rabbis made up laws afterwards is an illogical assumption. At which point would they have told the Jews that one cannot plant on Shabbos? Why would the Jew listen to them? He would say I have always planted on Shabbos. At which point were the details made up, and why would the people have followed them? We are a stiff-necked nation. Rabbi Sa’adiah Gaon (8th century Babylonian sage) explains that we are Jews by virtue of the Torah Hashem gave us. On Shavuos we will celebrate the written and oral law that were given to us by Hashem at Har Sinai, 3300 years ago.