“The triune God made a decision – a decision of humiliation… This decision carried with it no necessity; it was not necessary for the second person of the Trinity to decide to humble himself. He had every right to refrain from such a decision and to not add to himself the humiliating status of humanity. But he determined not to. This second person – one who was equal to God, who is in the form of God, who is himself God (John 1:1) – did not stop being God (such a thing would be impossible), but rather he took on something that was not a part of his essential character previously. He took on human nature (John 1:14).
To be clear, Christ does not become the opposite of himself by taking on human nature. Moreover, it is not as though he gives up deity in order to become man. This pattern is nowhere given in Scripture; it is, as we have said, an impossibility (given what we understand of God’s essence). Rather, just as the “I AM” remains Lord while coming down to be the God of Abraham, Isaac, and Jacob, so the second person of the Trinity remains God, while coming down to assume human nature and therefore becomes the God-man. This, as we have said, is the covenant; as the Westministers Confession reminds us, Christ is the substance of the covenant. It is covenant condescension, inconceivable to comprehend fully, but nevertheless central to a basic understanding of God and his relationship to creation.”
K. Scott Oliphint, in Reasons for Faith: Philosophy in the Service of Theology (P&R Publishing Company 2006), page 242.