When did Judaism begin? The beginning was in two stages: A) the Exodus from Egypt - 'Yetzias Mitzraim', for which we celebrate Pesach; followed 50 days later by B) the Giving of the Torah - Matan Torah (Revelation at Sinai), for which we celebrate 'Shevuos'; together forming the foundation and establishment of the Jewish people as a nation (see Ex. 19:56; Deut. 26:16-19). This was best expressed in the classical statement by Rav Saadia Gaon, C. 900 C.E.) (Emunos Vedeos 3:7) "We are a Nation only by our Torah", meaning that we are a nation not by the sharing of a common land, language, history or culture, etc. as are all other nations, but by the uniqueness of having been given G-d's law.

Why was it necessary for Judaism to begin in two stages? The answer is Sefiras-haOmer (counting of the weeks); the connection of Pesach to Shevuos. - The Kabbalists tell us that the seven week interim period between Pesach and Shevuos has a symbolic character of Chol-haMoed, the interim days between the first and last days of Pesach itself, (Ramban to Lev. 23:36; Rab. Bachya ibid 16). This means that Pesach and Shevuos are not simply two separate holidays, but they are in a deeper sense like the beginning and the end of one and the same holiday. How is this to be understood?

Throughout Tanach (the Bible) and Chazal (the Rabbis of the Talmudic period), the relationship of Hashem to the Jewish people is compared allegorially to that of husband and wife, as we know from Shir HaShirim (the Song of Songs) which we read on Pesach. The Prophet Jeremiah says (2:2), "Thus says the L-rd, I rememberthy love as a bride, when thou didst go after Me in the wilderness etc." In the Mishna (Taanis 4:8), the verse "yom chasunaso", (SH''S, 3:11), the day of His (G-d's) marriage, is referred to as yom Matan Torah, the day of Shevuos. So Pesach was the betrothal (erusin) and Shevuos was the marriage (nesuin) with the natural period of yearning in between. This idea goes further. Pesach is the time when the Jews separated themselves from the sheep (seh) worshipped by Mitzraim (Ex. 8:22), (as for example, today the cow is considered sacred to the Hindus in India). The Mishna (Shabbat 9:1) says that idols, avoda zara , have the tumah - defilement of nida, a menstruant woman. A nida must count seven days before immersing in the mikvah (ritual bath) for her tahara - purification, to her husband. So, here too, after the Jews rejected idolatry on Pesach, they needed seven weeks (not just days, because this was a whole people) of counting to become purified to accept the Torah. And then they did the ritual immersion - tevilah, to be purified for Matan Torah, as stated in the Gemara, (Yevamos 46b). These then are the two parts of the beginning of Judaism, the final Covenant between G-d and His people - His Nation, allegorially composed of the betrothal and marriage, Pesach and Shevuos, with the interim period connecting the two.

On Pesach, we bring the sacrifice of the (Korban ha) Omer, from barley (seorin), basically an animal food. On Shevuos, we bring the Korban Shtei HaLehem (two Breads), from wheat (chitin), the basic food staple of human beings, not animals (Mishna Sota 14a: Menachos 76b). This is to symbolize, that just being freed from slavery, the story of Pesach, makes us free, but only like animals are free in the jungle instead of a cage in the zoo, with no real purpose. But when we used this freedom of choice, free will, to accept the Torah, we then became real human beings, with a higher purpose in life, not just to eat and drink like animals, but in order to serve HaShem. As we quoted above from Rav Saadia Gaon, "We are only a Nation by our Torah", the Divine Instructions for life. This fits in to the previous idea as well, because when the bride becomes the wife to her husband, only then does she find real fulfillment and satisfaction, with purpose and meaning in life.