In today's first reading we have Christianity's most celebrated passage from the prophet Ezekiel. Led by God, the prophet witnesses a vision of the resurrection of Israel. He sees a whole plain of dry bones transformed and reborn into a "vast army," inhabited by the Spirit of God.
Note God's promise to Israel: I will put my spirit within you that you may live, and I will settle you upon you land; thus you shall know that I am the Lord. I have promised, and I will do it, says the Lord.
The prophecy is fulfilled in the mystery of Christmas. Jesus Christ is conceived by the Holy Spirit and thus his humanity becomes the means and the vehicle for God to put his Spirit into all of humanity. Those who are willing to receive the Gift are settled within the Church, the New Jerusalem that comes down from God. (Revelation 21) Thus we come to the promised knowledge of God, for no one can say that Jesus is Lord, except by the Holy Spirit. (1 Corinthians 12:3)