Hebrews 7:3 presents Jesus as eternally preexistent in the past as well as eternal in the future.
Heb. 7:3 "He [i.e. Melchizedek] is without father or mother or genealogy, having neither beginning of days nor end of life, but resembling the Son of God he continues a priest forever."
The reason why Unitarians don't see the eternal preexistence of Christ in Heb. 7:3 is because they read it with their Unitarian colored glasses as teaching the Son of God was made to RESEMBLE MELCHIZEDEK. But that's the opposite of what it's saying. It DOESN'T say the Son of God was made to resemble Melchizedek. Rather it says Melchizedek was made to RESEMBLE CHRIST the Son of God. Yes, it's true that earlier in redemptive history Melchizedek was a type of Christ, while Christ was the later anti-type and fulfillment of the type of Melchizedek. Is this a contradiction to what I argued above? No, because even further back [i.e. earlier] than Melchizedek, the eternal Son of God was the archetype of Melchizedek.
As the type [Melchizedek] had the appearance of having no beginning, so Jesus the anti-type who is the reality and fullness of the type ACTUALLY had "neither beginning of days nor end of life". This is why the author could say Jesus Christ is "the same YESTERDAY, today and forever" [Heb. 13:8]. And why Ps. 102 which was originally about Yehovah/Yahweh could be applied to Jesus in Heb. 1:10ff. Attributing to Him the divine prerogative and characteristics of creation, immutability, eternality, aseity.
If the author of Hebrews only wanted to affirm the eternality of the Son of God moving forward in time, but not backward in time, he wouldn't have described Melchizedek [whom he says resembles the Son of God] as having the appearance of "having neither beginning of days nor end of life". Instead, the author of Hebrews would have only said Melchizedek had no apparent end of life [leaving out not having beginning of days].
The Jamieson, Fausset & Brown commentary on Heb. 7:3 states:
//"...When Alford denies that Melchisedec was made like the Son of God in respect of his priesthood, on the ground that Melchisedec was prior in time to our Lord, he forgets that Christ's eternal priesthood was an archetypal reality in God's purpose from everlasting, to which Melchisedec's priesthood was "made like" in due time. The Son of God is the more ancient, and is the archetype: compare Heb 8:5, where the heavenly things are represented as the primary archetype of the Levitical ordinances. ..."//
https://biblehub.com/commentaries/hebrews/7-3.htm
Just to make it clear, I think Melchizedek was a normal human being who did have parents and possibly even descendants. But that doesn't undermine my point. Melchizedek is said to have been made to resemble the Son of God. So, when he's described as appearing to not have had beginning of days or end of life, the author is implying that the Son of God literally had no beginning of days or end of life. Because Melchizedek was made to appear like and resemble the Son of God.
When dealing with Unitarians, it's sometimes necessary to point out the following:
When the text says, Melchizedek was made to resemble the Son of God. How does the text say he resembles the Son of God? By having the appearance [though not the reality] of having no beginning of days or end of life. Meaning, the Son of God actually had no beginning of days or end of life. Again, when you recognize that difference between X resembling Y vs. Y resembling X, you'll see that the author is saying Jesus really has what Melchizedek only had the appearance, but not the reality of. Namely, beginninglessness and endlessness.
