The angel said to her, "The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the child to be born will be holy; he will be called Son of God. (Luke 1:35 NRSV)
Proponents of Socinian (so-called "Biblical unitarian" [such a Christology is not based on sound biblical exegesis]) often appeal to this passage as "proof" that Luke did not believe Jesus personally pre-existed his miraculous conception. Anthony Buzzard, for instance, focuses on "therefore" (διο και), arguing that it means "for this reason," hence showing that Jesus being called "Son of God" is due to his being conceived miraculously, and not due to being "Son of God" in the sense of pre-existing eternally; instead, he came into existence only 2,000 years ago.
Such an argument ignores that there are different applications of the term “Son of God” in the Bible. In this verse, it is being used of Jesus as the Messiah. The role of “Messiah” (anointed one; χριστος) requires one to be human, which the verse teaches, although it does not preclude personal pre-existence (this is where Latter-day Saint theology, unlike Trinitarianism, can affirm the true humanity of Jesus consistently).
To quote one scholar whom Anthony Buzzard is fond of:
The title “Son of God,” like “Son of Man,” was a recognized designation of the Messiah. In Enoch, and often in 4 Ezra, the Almighty speaks of the Messiah as His Son. Christ seldom used it of Himself (Mt. 27:43; Jn. 10:36). But we have it in the voice from heaven (3:22, 9:35); in Peter’s confession (Mt. 16:16); in the centurion’s exclamation (Mk. 15:39); in the devil’s challenge (4:3, 9); in the cries of demoniacs (Mk. 3:11, 5:7). Very early the Christian Church chose it as a concise statement of the divine nature of Christ. See on Rom. 1:4, and Swete, Apost. Creed, p. 24. For ἂγιον see on Rom. 1:7. The radical meaning is “set apart for God, consecrated.” (Alfred Plummer, A Critical and Exegetical Commentary on the Gospel According to S. Luke [London: T&T Clark International, 1896], 25)
Furthermore, in Luke 13:34 (cf. Matt 23:37), we read:
O Jerusalem, Jerusalem, which killest the prophets, and stonest them that are sent unto thee; how often would I have gathered thy children together, as a hen doth gather her brood under her wings, and ye would not!
A number of scholars have discussed this text in Luke and Matthew, and have argued that this teaches the personal pre-existence of Jesus, as it is talking about actions undertaken by Jesus, not during his mortal lifetime, but pre-mortal existence. For a discussion, see Simon J. Gathercole, The Pre-Existent Son: Recovering the Christologies of Matthew, Mark, and Luke (Eerdmans, 2006). While I believe that Gathercole is often over-enthusiastic (and hence, engages in eisegesis) about certain issues in this volume, his exegesis of Matt 23:37/Luke 13:34 and his chapter refuting Wisdom Christology is spot-on in this volume. If such is the case, then it blows the arguments about Luke 1:35 out of the water.
The errant interpretation (again, eisegesis) provided by Socinians such as Anthony Buzzard about this verse is further refuted when one engages in any meaningful exegesis of John 8:58 and 17:5, and see how weak the attempted rebuttals to this passages truly are. A related text would be Heb 1:10-12. While I disagree with his Trinitarianism, Thomas Farrar (a former Christadelphian) has an excellent paper on Heb 1:10-12 and how it shows the personal pre-existence of Jesus:
You, Lord, in the beginning: Hebrews 1:10-12 and Christology (IMO, this is the single best New Testament passage affirming the pre-existence of Jesus)
It interacts with the various Socinian attempts to "answer" (e.g., those of Andrew Perry and Ron Abel) this passage, showing how desperate they truly are.
The New Testament affirms the personal pre-existence of Jesus. While Christadelphians, members of the Church of God Abrahamic Faith, and other movements believe such to be heresy, sadly, they are guilty of proclaiming a false conception (pun intended!) of Jesus