I learned this year (from Wikipedia, of all places) that the Jewish holiday corresponding to Pentecost commemorates the giving of the law on Mt. Sinai. (See Acts 2 for the Christian Pentecost story if you’re unfamiliar with it – it is when the Spirit first comes down upon the new church.) This makes sense, of course, since Easter corresponds to Passover and the deliverance of Israel from Egypt!
But it struck me as very fitting that the Holy Spirit should come down to indwell the church on the same day that God had given His law to His people, in light of the prophet Jeremiah:
“Behold, the days are coming, says the LORD, when I will make a new covenant with the house of Israel and the house of Judah, not like the covenant which I made with their fathers when I took them by the hand to bring them out of the land of Egypt, my covenant which they broke, though I was their husband,” says the LORD. “But this is the covenant which I will make with the house of Israel after those days,” says the LORD: “I will put my law within them, and I will write it upon their hearts; and I will be their God, and they shall be my people.”
– Jeremiah 31:31-33
The external law, which was the binding condition of the old covenant, has been replaced in the new covenant by an internal law: by the direction and transforming power of the Holy Spirit living within God’s people – and so while the old covenant was broken by Israel’s disobedience, the new covenant can be fulfilled through the grace and power of God Himself, since He is now on both sides of the relationship. We no longer need to follow a list of rules, but rather to abide in Christ (which is not less, but more, than mere obedience), and it is the Spirit abiding within us that renders it possible.
And that great new hope, the beginning of the fulfillment of the wild promises of God to a wayward people, is why we celebrate Pentecost.