The Christian meaning of the term “deity of Christ” is fairly clear. The Christian believes that there is a personal God, Creator and Ruler of the universe, the God who is infinite, eternal, and unchangeable. So when the Christian says that Jesus Christ is God, or when he says that he believes in the deity of Christ, he means that that same person who is known to history as Jesus of Nazareth existed, before he became man, from all eternity as infinite, eternal, and unchangeable God, the second person of the holy Trinity.
J. Gresham Machen, The Person of Jesus: Radio Addresses on the Deity of the Savior (Philadelphia: Westminster Seminary Press, 2017), 18.