When they hear about the eternal Sonship, unitarians tend to roll their eyes and think, "There they go again with their unscriptural jargon." At least, that's what I did when I was a unitarian. However, my studies of the Word of God have led me to the conclusion that the eternal Sonship is a biblical idea. One of the strongest pieces of evidence for this is found in Hebrews 7:1-3:
"1 For this Melchizedek, king of Salem, priest of the Most High God, who met Abraham as he was returning from the slaughter of the kings and blessed him, 2 to whom also Abraham apportioned a tenth part of all the spoils, was first of all, by the translation of his name, king of righteousness, and then also king of Salem, which is king of peace. 3 Without father, without mother, without genealogy, having neither beginning of days nor end of life, but made like the Son of God, he remains a priest perpetually." (NASB)This text doesn't come up a lot in Christological debates or discussions because the argument the writer of Hebrews is making is rather cryptic. Once the idea is unpacked, however, the implications for our understanding of Christ are unmistakable. One recent commentary on Hebrews explains the point very well:
"In Genesis 14, Melchizedek is introduced out of the blue and disappears as quickly, his brief appearance interrupting a narrative that deals with the king of Sodom (vv. 17, 21-24). We know his name and office but nothing of his family or his previous or subsequent history. In terms of the biblical narrative, he is thus ‘without father or mother, without genealogy, without beginning of days or end of life.’ His literary persona therefore suggests to our author a parallel with the Son of God, who is in very fact ‘without beginning of days or end of life,’ and the psalm, which speaks of ‘a priest forever in the order of Melchizedek,’ reinforces this thought. It is not a historical argument – nothing in the OT suggests Melchizedek historically had no parents, was not born, and did not die, and our author’s argument does not necessarily show that he thought this to be the case (though v. 8 may point that way). It is rather an argument from literary silence, setting Melchizedek up as a literary model for the eternal Son of God. In his very rootlessness and timelessness, he forms a suitable model for the one who was to come, the Son (not of any man but) of God, who shares God’s eternal existence and can thus uniquely exercise that eternal priesthood Psalm 110:4 has claimed to be the prerogative of ‘the order of Melchizedek.’ To our historically tuned minds, this may seem a bizarre conclusion to draw from silence, but it is an argument from the text, not from history, following the well-attested Jewish hermeneutical principle (found both in rabbinic and in Alexandrian writings) that what is not mentioned in the Torah does not exist. The comparison serves to underline the eternity of our true high priest in contrast with the transience of the OT priests, a theme that will be developed in vv. 23-25." (R. T. France, Hebrews, The Expositor’s Bible Commentary, Hebrews - Revelation, ed. Longman and Garland, p. 92)The comparison made by the writer of Hebrews cannot merely refer to the Son of God in his humanity, because as a human being Christ did have a genealogy, a beginning of days and a mother. It is also worth noting that the writer likens Melchizedek to the Son of God, and not the other way around. This would be anachronistic unless the Son pre-existed (cf. John 1:30; 8:58).
Nor can this argument from Heb. 7:3 be dismissed as case of "proof texting." It must be seen in the fullness of the message of the writer to the Hebrews, who describes Christ as actively existing in the (distant) past, present and future. Past: the world was made through him (Heb. 1:2). Present: he upholds all things by the word of his power (Heb. 1:3). Future: he is the heir of all things (Heb. 1:2).
Past: He laid the foundations of the earth (Heb. 1:10). Present: he remains (Heb. 1:11). Future: his years will have no end (Heb. 1:12). Past, present and future: "Jesus Christ is the same yesterday, and today, and forever" (Heb. 13:8).
Thus we should not be surprised to see that Heb. 7:3 implicitly teaches the eternal Sonship of Christ. It is hardly an anomaly; it fits right into the picture of Christ painted throughout the epistle.