How was Jesus fully God and fully man?

The opposite error to saying that He is two persons is to say that His two natures have combined into one. It was an ancient heresy that the two natures, Manhood and Deity, merged together. For this reason the writer does not like the expression "the God-man". It implies to him (though not to many orthodox Christians) that He is half man and half God. Such an idea must be resisted. He is wholly Man and wholly God. He possesses all the attributes and properties of real, unfallen, holy Manhood, and at the same time possesses all the infinite attributes and properties of Deity. The unity consists, not in the amalgamation of essence, but in the Oneness of His Person.

Let it always be remembered that it was not God dwelling in a human body — which is one of the many heresies about the person of our Lord. Nor is it even God dwelling in a perfect man — with body, soul and spirit — as though He could or would finally be separated from this manhood. But it is "the Word became flesh." He became identified with perfect humanity, took up humanity so completely that there was but one person, the blessed Son of God.

His humanity was not merely in form, something put on, as one would put on a garment for a certain purpose and then throw it off again when that purpose was achieved. It was a reality. He became an actual man, and this He will never cease to be.

"The Holy Spirit will come upon you and the power of the Highest will overshadow you; therefore also the Holy thing which will be born will be called the Son of God." One has said, "The Holy Spirit would come upon her, would act in power upon this earthen vessel, without its own will, or the will of any man… God acting upon Mary was the divine source of His existence on the earth as Man." He was not man innocent, still less man fallen. He was Man holy.

Look, for example, at Christ on the sea of Galilee. While the storm was raging "he was asleep on a pillow" — precious display of His perfect manhood! But in a moment He rises from the position of real humanity into all the dignity and majesty of Godhead, and, as the supreme Governor of the universe, He hushes the storm and calms the sea. There is no effort, no haste, no bracing Himself for the occasion. With perfect ease He rises from the condition of real humanity into the sphere of essential Deity. The repose of the one is no more natural than the activity of the other. He is as perfectly at home in the one as in the other.

If He had not become man He could not have been our substitute and representative… and if He had not been the sinless Man He could not have stood for us, for how could one who had sins of his own to suffer for be a substitute for others? But if He had not been more than man, if He had not been God in His own eternal being, His sacrifice would have had no atoning value. Who could measure and meet the claims of God's eternal justice but God?… If He had not been man He could not have died; if He had not been God His death would have been without value.

"No one knows the Son except the Father" is a wonderful statement, settling once and for all the thus-far-and-no-further of our knowledge in this direction. Nearly all the great heresies that have torn the Church of God apart from the time of Arius onward have started in faulty and speculative theories about the truth of Christ's Person. Our only sure course is to hold to the very words of Scripture, and to refuse speculation into mysteries not revealed.