The Word did not stop being God and turn into a man instead.

Rather, he remained God and acquired or assumed to himself a human nature, which is to say, a human body and soul. He didn't change into something else, but added to himself. That's an important distinction to make.

If we insist that God stopped being God when he became a man, then we reject the truth that he is the great "I AM." Instead we make him out to be the "I USED TO BE."

If that were the case, he wouldn't have been truly God from the start -- just like using a pencil eraser to erase permanent ink would mean it wasn't really permanent ink to begin with.