Blessed are the poor in spirit, for theirs is the kingdom of heaven. (Mt 5:3)
The first of the Beatitudes is that which highlights the basis for all the other seven in St. Matthew’s Gospel (Mt 5:3-12) In fact, God Himself made himself poor in order to save us. The incarnation of Christ can be seen as a impoverishment. Divesting Himself of the accidents (used here in the philosophical sense) of divinity, God became a human being. St. Paul expresses this in his letter to the Philippians:
… though he was in the form of God, did not count equality with God a thing to be grasped, but emptied himself, taking the form of a servant, being born in the likeness of men. And being found in human form he humbled himself and became obedient unto death, even death on a cross.
The Son of God went beyond just emptying Himself of His divinity. In His humanity, He even emptied Himself of human life when as a consequence of obedience to His Father, He was nailed on a cross and died.
The first Beatitude has a very special meaning for a priest celebrating Mass. Whilst I retain my personality when celebrating Mass, I am aware that I become Christ during the celebration. For the sacrifice of the Mass is that one sacrifice of Christ on the Cross, where Christ is both Priest and Victim (see Hebrews 9). When I lift chalice and paten at the Doxology of the Eucharistic Prayer, it is not I who offer Christ the Victim to the Father, it is Christ. If I am full of myself during the celebration, I cannot be truly alter Christus. In spirit, I must be poor, almost like emptying myself so that I can be filled with the person of Christ. Only when I am truly poor in spirit will I be really aware of the great privilege granted to me to celebrate the Eucharist.