CCC 526

To become a child in relation to God is the condition for entering the kingdom.²⁰⁵ Cf. Mt 18:3-4. 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".²⁰⁶ Jn 3 7; 1:13; 1:12; cf. Mt 23:12. Only when Christ is formed in us will the mystery of Christmas be fulfilled in us.²⁰⁷ Cf. Gal 4:19. 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.²⁰⁸ LH, 1 January, Antiphon I of Evening Prayer.