The Christmas mystery
To become a child in relation to God is the condition for entering the kingdom. For this, we must humble ourselves and become little. Even more: to become "children of God" we must be "born from above" or "born of God". Only when Christ is formed in us will the mystery of Christmas be fulfilled in us. Christmas is the mystery of this "marvellous exchange":
O marvellous exchange! Man's Creator has become man, born of the Virgin. We have been made sharers in the divinity of Christ who humbled himself to share our humanity.
Source: Catechism of the Catholic Church (Vatican.va)