He that loveth me shall be loved of my Father; and I will love him and will manifest myself to him.—John 14, 21.
The feast of the Nativity is the feast of the birth of the Word; his eternal generation in the bosom of the Father, his temporal birth; and his mystical birth in the souls of men, in which is involved their birth to divine life.
The Christmas liturgy is at different times concerned with each of these aspects of the Mystery.
Dominus dixit ad me. Filius meus es tu, ego hodie genui te, is the triumphal Introit to Midnight Mass.