The Trinity before the birth of Jesus If Jesus was born at a certain date in history, do trinitarian Christians believe that the Trinity did not exist before that date? Alternatively, is some sort of incorporeal Jesus believed to have been in existence despite not yet having been born?
Christians believe that the Trinity consists of three distinct persons with one divine nature. 

This Trinitarian Godhead exists infinitely and eternally, and therefore existed before time. 

Christians also adhere to the very distinct doctrine of the Hypostatic Union, which is the belief that Jesus Christ has two natures. 

One nature is truly divine. One nature is truly human.

These two doctrines are clearly stated in the first chapter of the Gospel of St. John:


  In the beginning was the Word, and the Word was with God, and the Word
  was God. He was in the beginning with God.


The "Son" (2nd Person of the Trinity) existed "before all ages."  The Son was born into the world of a virgin roughly 2000 years ago. His name is Jesus Christ.

The Nicene Creed puts it best:


  We believe in one Lord, Jesus Christ, the only Son of God, eternally
  begotten of the Father, God from God, Light from Light, true God from
  true God, begotten, not made, of one Being with the Father. Through
  him all things were made. For us and for our salvation he came down
  from heaven: by the power of the Holy Spirit he became incarnate from
  the Virgin Mary, and was made man. For our sake he was crucified under
  Pontius Pilate; he suffered death and was buried. On the third day he
  rose again in accordance with the Scriptures; he ascended into heaven
  and is seated at the right hand of the Father. He will come again in
  glory to judge the living and the dead, and his kingdom will have no
  end.


Was the Son eternally begotten?