eternal sonship
The teaching that the second person of the Trinity has existed eternally as the Son, so that the relationship between God the Father and God the Son has always been an eternal Father-Son relationship.

• From scripture: 
  

  He is the image of the invisible God, the firstborn of all creation. 16 For by him all things were created, in heaven and on earth, visible and invisible, whether thrones or dominions or rulers or authorities—all things were created through him and for him. (Colossians 1:15-16 ESV)

  …but in these last days he has spoken to us by his Son, whom he appointed the heir of all things, through whom also he created the world. (Hebrews 1:2 ESV)

• From the Belgic Confession:

  Article 10 That Jesus Christ is true and eternal God.

  We believe that Jesus Christ, according to his divine nature, is the only begotten Son of God, begotten from eternity, not made nor created (for then he should be a creature), but co-essential and co-eternal with the Father, the express image of his person, and the brightness of his glory, equal unto him in all things. He is the Son of God, not only from the time that he assumed our nature, but from all eternity, as these testimonies, when compared together, teach us. Moses saith, that God created the world; and John saith, that all things were made by that Word, which he calleth God. And the apostle saith, that God make the worlds by his Son; likewise, that God created all things by Jesus Christ. Therefore it must needs follow, that he, who is called God, the Word, the Son, and Jesus Christ, did exist at that time, when all things were created by him. Therefore the prophet Micah saith, His goings forth have been from of old, from everlasting. And the apostle: He hath neither beginning of days, nor end of life. He therefore is that true, eternal, and almighty God, whom we invoke, worship and serve.

• From A Defense of the Eternal Sonship of Christ by David Dunlap:

What saith the Scriptures? God’s word must be the final authority for all we teach and believe. In regard to these errors, do the scriptures teach that the Lord Jesus Christ was the eternal Son of God before the incarnation? The answer must be resounding YES. The Old Testament is full of statements which verify His Sonship. Isaiah states, “For unto us a child is born, unto us a Son is given…” (Isa. 9:6). In another place Agur writes, “Who has gathered the wind into his fists? Who hath established all the ends of the earth? What is His name, and what is His Son’s name, if thou canst tell” (Prov. 30:4).

The Scriptures teach that it was not at the incarnation that Christ became the Son, but that He was always the Son of God. …

The Son created the world. Both Colossians 1:13-19 and Hebrews 1:2 state that the Lord Jesus Christ is the eternal Son, before His incarnation, when He created the worlds.

The Father sent the Son - Time and again the scriptures state that the Father sent the Son. Since the Father sent the Son, it must follow that the Lord Jesus Christ was the eternal Son before He came into the world. (Isa. 9:6; John 3:17)

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The Son had a relationship with the Father from eternity past. “I came forth from the Father, and am come into the world … and go to the Father”