The giving of the Torah on Mount Sinai brought about a fundamental change in the relation of the physical world to G-dliness. As our sages say: “Even though G-d decreed that ‘the heavens belong to G-d and the earth was given to mankind,’ when the Torah was given this decree was cancelled.”
In other words, before the Torah was given there was no possibility to connect the spiritual and the physical. If a physical object was used for a mitzvah, it did not in itself become holy, but remained the same physical object. Only after the Torah was given on Sinai was there a possibility of elevating a physical object and making it holy, through using it for the purpose of a mitzvah.
In addition to receiving the Torah, there were two additional events that occurred on Shavuot: the passing of King David, and the passing of Rabbi Israel Baal Shem Tov, the founder of Chassidut. These two figures had a profound impact on the process of transforming the profane into holiness, as described above.
The life work of King David was to prepare the ground for building the Holy Temple (which was completed by his son, King Solomon). The Temple is a physical structure in which G-d dwells. More than any other place on earth, it represents the fusion of the physical and the spiritual. The Baal Shem Tov, through his teachings, revealed to the public that each of us has a spark of G-d within us that sustains us and gives us life at every moment. We may be physical beings but we are hosts to the Divine! Through being mindful of our great power we can complete the process of perfecting the world and making it holy, which will culminate with the coming of Moshiach.