“The Word became flesh” (John 1.14). The verb “became” [egeneto] here does not entail any change in the essence of the Son. His deity was not converted into our humanity. Rather, he assumed our human nature. Through the most inconceivably intimate personal union of the Logos with our humanity, the particular Jewish male, Jesus of Nazareth, was conceived by the Holy Spirit in the virgin’s womb. Therefore, the incarnation does not represent a fusion of pre-existing natures, much less of pre-existing persons, but the eternal Son’s assumption of humanity. Strictly speaking, then, His humanity is impersonal. There is a union of the person of the Son with our humanity, not of a divine and a human person. Each nature is entirely preserved in its distinctness, yet in one and the same person.
This assumption of our complete humanity need not include sin, of course, because sinfulness is accidental, rather than essential, to our nature. Since Jesus Christ was, in the words of the Belgic Confession, “conceived in the womb of the blessed virgin Mary by the power of the Holy Spirit, without male participation” (Belgic Confession 18), His humanity was untainted by original sin (Hebrews 4.15; cf. Isaiah 7.14; Matthew 1.18-20; Luke 1.34-35).
From: The Christian Faith: A Systematic Theology for Pilgrims on the Way by Michael Horton (Grand Rapids: Zondervan, 2011), p. 468.