This is the Introit for the Mass of the Nativity of the Lord during the Night.
The Lord hath said to me: Thou are My Son, this day have I begotten Thee. -- (Ps.2. 1). Why have the Gentiles raged, and the people devised vain things? V.: Glory be to the Father , and to the Son, as it was in the beginning, is now, and will be forever. . . -- The Lord hath said to me . . .