This Parsha begins with the words, “And G-d spoke to Moshe at Mt. Sinai”. The narrative then proceeds to a discussion of the requirement to refrain from all agricultural activity in the Land of Israel every seven years. The commentaries are puzzled by the unusual reference to Mt. Sinai only in regard to this mitzvah since all of the mitzvot were given at Mt. Sinai.
Abarbanel explains that even though Moshe received all the commandments at Mt. Sinai, he did not teach all of them to the people at the same time. Rather, he was instructed by G-d prophetically when to teach particular commandments.
When Moshe first descended from Mt. Sinai he was confronted by the tragedy of the idolatry of the golden calf. In order to rebuild the spiritual level of the nation, the emphasis of the Torah narratives for the rest of the Book of Exodus and the first several parshiot of the Book of Leviticus is on the spiritual purification of the nation, focusing on the construction and services of the Tabernacle, which represents the epitome of the pure spiritual relationship between G-d and the Jewish People, as well as numerous commandments focusing on the spiritual elevation of the people. As a result, G-d speaks to Moshe in the Tent of Meeting in the Tabernacle itself.
Once the solid foundation of sanctity has been rebuilt from its source in the Tent of Meeting or Tabernacle, the Torah can then again refer to Mt. Sinai, the original source of all the commandments. The ultimate relationship between G-d and the Jewish People can only be brought to fruition in the Land of Israel. However, the Torah makes it abundantly clear that only when the nation has been purged of its idolatrous and heretical past through its forty-year experience with the Tabernacle in the desert can it merit the privilege of dwelling in the Land of Israel. It is in the Land of Israel that the Jewish People are given the opportunity to demonstrate to the entire world their faith and trust in G-d as the ultimate source of our material welfare by refraining from agricultural activities every seven years.