5 If then thou hear the Gospel saying, The book of the generation of Jesus Christ, the Son of David, the Son of Abraham, understand “according to the flesh.” For He is the Son of David at the end of the ages, but the Son of God Before All Ages, without beginning. The one, which before He had not, He received; but the other, which He has, He has eternally as begotten of the Father. Two fathers He has: one, David, according to the flesh, and one, God, His Father in a Divine manner. As the Son of David, He is subject to time, and to handling, and to genealogical descent: but as Son according to the Godhead, He is subject neither to time nor to place, nor to genealogical descent: for His generation who shall declare? God is a Spirit; He who is a Spirit has spiritually begotten, as being incorporeal, an inscrutable and incomprehensible generation. The Son Himself says of the Father, The Lord said unto Me, You are My Son, today have I begotten You. Now this today is not recent, but eternal: a timeless today, before all ages. From the womb, before the morning star, have I begotten You.
Source: Catechetical Lectures (New Advent)