The Trinity before the birth of Jesus

Just to add to the great answers by both David Stratton and Charles Alsobrook, That Jesus was both fully man and fully God is central to Christianity. Unless Jesus was a sinless man He would not be an acceptable sacrifice for our sins. That could only be possible if he were also deity, which by definition would be an eternal being.

There are, of course, exceptions, but most of mainstream Christianity believes that Jesus was eternal, that He always existed, coequal with God. The Trinity is God, and is Eternal.

Addressing Jesus' pre-existence:

Scripturally, the strongest support for this belief is John 1.

John 1 King James Version (KJV)

1 In the beginning was the Word, and the Word was with God, and the Word was God.

2 The same was in the beginning with God.

3 All things were made by him; and without him was not any thing made that was made.

For some documentation showing that various mainstream denominations believe this...

Grace communion International, a Protestant denomination says this:

The pre-existence of Christ is clearly stated in John 1:1-4: "In the beginning was the Word, and the Word was with God, and the Word was God. He was with God in the beginning. Through him all things were made; without him nothing was made that has been made. In him was life…" It is this Word or Logos in Greek who became incarnate in Jesus. "The Word became flesh and made his dwelling among us," John tells us (verse 14).

The eternal, uncreated Word who was God, and yet was with God as one of the Persons of the Godhead, became a human being. Note that the Word "was" God but "became" a human being. The Word never came into being, that is, he didn’t "become" the Word. He always was the Word, or God. The Word’s existence is open-ended. He has always existed.

Caltholictruth.com says this:

Bethlehem and Nazareth were not the first homes of Jesus; His home was from all eternity in the bosom of the Father. "In the beginning," says St. John (John 1:1), "was the Word, and the Word was with God, and the Word was God." The Word Whom Scripture also calls Son of God proceeds from the Father by intellectual generation; He is the Father's understanding of Himself. He is the first Word ever spoken, and is in all things equal to Him by Whom He was spoken. The Father Who uttered Him is not prior to the Word, but both are equally eternal.

Yes, the Incarnate Christ is older than the oldest creatures since He is at once their Creator and the Pattern after which they were created: "All things were made by Him: and without Him was made nothing that was made" (John 1:3); "In Him were all things created in heaven and on earth, visible and invisible, whether thrones, or dominations, or principalities, or powers; all things were created by Him and in Him. And He is before all, and by Him all things consist" (Colossians 1:16, 17). In the beatific light of the heavenly palace, says St.Paul, the Word before His Incarnation dwelt in the "form of God," that is in a nature which - because it is Divine - cannot be acquired but is eternal (Philippians 2:6-8). Verily Christ could say of Himself, "Before Abraham was made, I am" (John 8:58).

A Google Search of "Jesus before incarnation" will turn up thousands of similar statements from myriad Churches and denominations. Of course, it would be inaccurate to say that all Christians believe this, it is a demonstrably widely held belief.

The details of what His pre-existence is like is unknown. However, most also believe that He made appearances in the Old Testament long before His physical birth. (This is known as a Christophany)

The LDS Church gives an entirely different view, still holding that He existed, but in the "Premortal life" that they believe.

Premortal existence of Christ

• The Lord appeared unto Abram:Gen. 12:7; ( Gen. 17:1; Gen. 18:1; Abr. 2:6–8; )
• The Lord spoke unto Moses face to face:Ex. 33:11; ( Deut. 34:10; Moses 1:1–2; )
• I saw the Lord standing upon the altar:Amos 9:1;
• In the beginning, the Word was with God. And the Word was made flesh, and dwelt among us:John 1:1, 14; ( 1 Jn. 1:1–3; )
• Before Abraham was, I am:John 8:58;
• Glorify thou me with the glory which I had with thee before the world was:John 17:5;
• Isaiah verily saw my Redeemer as I and my brother Jacob have seen him:2 Ne. 11:2–3;
• On the morrow come I into the world:3 Ne. 1:12–14;
• Christ was before the world began:3 Ne. 26:5; ( John 6:62; )
• As I appear unto thee will I appear unto my people in the flesh:Ether 3:14–17;
• Enoch saw the Lord and walked with him:D&C 107:48–49;
• My Beloved Son, which was my Beloved and Chosen from the beginning:Moses 4:2;
• The Lord said: Whom shall I send? And one answered like unto the Son of Man: Here am I, send me:Abr. 3:27;

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?