Did God, in some way stop being God when He took a human nature?
It is an important question.
No! When God spoke to Moses in the burning bush, had He ceased to be God for that? Of course not! God can not be limited by anything. When God revealed himself in with a human nature—a human body and a human soul, He was not at all limited by this humanity. He continued to reign over this universe, He continued to be what He always was (God), but in Jesus He became what He was not before (man). However, it must be clear that Jesus was not 50% God and 50% man. He was 100% God and 100% man, and continues to be both fully God and fully man.
Addition or subtraction?
The divine Person has taken upon Himself human nature. It would be impossible for us to understand perfectly the mystery of how this happened, but the principle is conceivable and can be accepted by faith. It is clear that God has the power to add to Himself a human nature, and to do so in such a way as to unite two natures into one Person without ceasing to be God.
An illustration will help us to better grasp the thought. Take for example a sparkling light and place it in a glass. Will the glass reduce the brightness of the light? No! On the contrary, because of its dazzling reflection in contact with the glass, it will only make the light shine more beautifully, with even greater brilliance and clarity. In the same way, when God became a man in the person of Jesus Christ, the body he took did not prevent him from continuing to be God, which he always was and will remain forever. Jesus said, “I am the light of the world.” God became what he was not before in the person of Jesus, a man, while continuing to shine with all his glory and to rule the universe. This is how He wanted to reveal himself even more clearly so that the world could know him.