Shavuot is one of the Shlosh Regalim, or Three Festivals. In times when the Holy Temple still stood in Jerusalem, the Israelites made pilgrimage to the Temple for these three holidays. In those times the traditions of the holidays were somewhat similar.
The Festival of Shavuot (Pentecost) is traditionally held as the anniversary of the giving of the Torah as well as the giving of the Holy Spirit. As part of the commemoration, the Ten Sayings (Commandments) are often read publicly in the Assembly on the Feast of Shavuot.
According to the traditional reckoning, the first commandment is simply: “I am the L-RD your G-d who brought you out of the land of Egypt, out of the house of slavery.” (Exodus 20:1-2)
The first of the Ten Commandments is to believe in G-d and to accept His claim of ownership over us. Unless we accept the basic premise of G-d’s existence, no number of commandments will have any authority or meaning for us. The need for such a command demonstrates the incongruity of ascribing to morality without believing in a moral authority. If we accept the premise of objective morality, we must accept that there is a higher authority issuing that objective standard.
We also learn from this that redemption must precede commandments. Before we can receive the Torah, we must already be “redeemed.” Israel’s salvation from Egypt was not accorded to her on the basis of her obedience to the commandments. She was already redeemed (on no merit of her own) before the Torah at Sinai were given to her. Thus, the first declaration at Sinai is a reminder of her redeemed status. Only because she is already redeemed is she able to receive the commands of God.
True legalism continually attempts to reverse this process by claiming that one’s obedience to certain commandments (or all the commandments) is the mechanism by which salvation is earned. The first of the Ten Commandments directly contradicts this notion. Salvation and relationship with G-d precedes the Torah. We obey His Torah because Yeshua has first redeemed us, not the other way around.