"Even as he chose us in him before the foundation of the world, that we should be holy and without blemish in his sight in love. He predestined us to be adopted through Jesus Christ as his sons according to the purpose of his will..." (Ephesians. 1, 4). God did not think Adam, and then think Christ. First He thought Christ, and then, before the creation of the world, he thought us, predestining us in Christ.
Vatican II: Gaudium et spes #22: “The truth is that only in the mystery of the incarnate Word does the mystery of man take on light. For Adam, the first man, was a figure of Him Who was to come, namely Christ the Lord.”
Jesus Christ is the Son of the living God Who received the human nature/body from the Virgin in time. But because He is the Creator-Person outside time, the assumption of the human nature/body exists outside time, and hence the enfleshed Son of the living God "pre-exists" the creation of the world.
Biblical Witness to the Idea of Preexistence: Christoph Schonborn
“Belief in the preexistence of Christ did not just ‘fall from heaven’’ even in Jewish belief there are a variety of concepts of preexistent realities. Thus, for instance, in the time of Jesus the idea was current that the Torah was preexistent that it was there ‘before the foundation of the world.’ The same is true of the Wisdom of God, of his Word, the Logos, and finally, also, of the Messiah himself. His name was being kept hidden beneath the throne of God. The boundaries between a purely ‘notional’ or ‘ideal’ preexistence in the thought of God and a ‘real’ preexistence are fluid. Even in relation to the son of man, we find the notion of his preexistence in apocalyptic literature. ‘According to a Jewish theologoumenon that was widely handed down, seven (or sometimes, six) things existed ‘before the creation of the world’ … the Torah, Penitence, the Garden of Eden, Gehenna, the Throne of Glory, the Sanctuary, and the Name of the Messiah.’
Rabbinic traditions concerning the preexistence of the Messiah are of particular interest here, for instance that in the midrash Pesikta Rabbati: ‘What is the evidence that the Messiah has existed since the beginning of the world’s creation? And the Spirit of God hovered.’ That is the messiah king! For it is said: ‘And the Spirit of the Lord shall rest upon him (Is. 11, 2).’ Therefore the Messiah is present at the creation of the world. It is equally striking that in Pes. R. 36:1, the original light of creation (Gen. 1, 4) is identified with the light of the Messiah that God is hiding beneath his throne. Who could fail to think of John 8, 12) ‘I am the light of the world’? The introduction of the idea of preexistence is thus not a startling invention, still less a Hellenistic overgrowth, but took place from an inner necessity.’… It is not the concept of preexistence in itself that is new and unusual, but the fact that the preexistent being ‘was in the form of God.’ Torah, Wisdom, Logos, and Messiah are created entities. Enoch and Elijah are creatures. But in Judaism, as soon as this fundamental dividing line is threatened, voices are raised in warning… It was not preexistence that was the problem, or even the sending of a heavenly, preexistent figure, his descent from the heavenly into the earthly realm. All of that was known to Jewish theology in the speculations concerning the Shekinah. The unusual thing about the Christian proclamation is that this preexistent Messiah and Son of man, Logos and Wisdom, Son of God, stands ‘no longer simply on the side of creatures, but at the same time also on the side of God.’ Judaism is, to be sure, familiar with heavenly entities who stand alongside God, such as personified characteristics of God like justice and mercy. In the Talmud, God prays that his mercy may get the better of his wrath. But that some personal entity, a preexistent Son of God, should himself be God, that is singular and new,”
 Footnote 20 of GS 22: Romans 5, 14: “(Y)et death reigned from Adam until Moses even over those who did not sin after the likeness of the transgression of Adam, who is a figure of him who was to come.” Tertullian: “The shape that the slime of the earth was given was intended with a view to Christ, the future man;” De Carnis resurrectione 6.
 See Martin Hengel, “The Son of God…” pp. 108-109.
 M. Hengel, “Son of God,” 119 (German).
 Christoph Schonborn, “God Sent His Son” Ignatius (2010) 71-73.