If Jesus Christ is a union of human and divine natures, was his human nature co-eternal with God, or only his divine nature? John 1:14 ("and the Word was made flesh") seems to imply that the Incarnation took place later. That, in turn, suggests that Jesus Christ’s human nature "happened" later—it was not present at the beginning, not co-eternal with God (except, perhaps, as a potentiality).
If so, then can it be said that the joint human-divine nature of Jesus Christ is the second person of the Holy Trinity? Or is only the divine nature of Jesus Christ (the Logos) the Second Person? How could Jesus Christ’s human nature be part of the Trinity if it is not co-eternal with God?
Jesus is the Eternal Word, the Eternal Logos, who is eternally begotten of the Father and equal to the Father. When he became incarnate by the Holy Spirit in the womb of his blessed Mother, he took on a human nature and entered time. His human nature had a beginning. We don’t want to create a problem where there is none. The fact that Jesus took on a human nature in no way diminishes the Holy Trinity. One of the Persons of the Holy Trinity, since the Incarnation, simply has a human nature as well as a divine one. If we said that his human nature supplanted his divine nature, we would have a problem. But such is not the case and could never be.