When we say that Jesus is God, we have to be very careful to qualify what we mean. We mean, when we say that Jesus is God, that Jesus has a divine nature—but He also has a human nature. Obviously, His human nature is not part of His deity; it’s a manifestation of His humanity.
There are two problems that arise when we deal with the question of the Trinity and the incarnation. The classic formulation for the Trinity is this: God is one in essence, but three in person, that is, the three persons of the Godhead—the Father, the Son, and the Holy Ghost. They are all, as one essence, fully God. They are co-eternal and co-substantial in terms of their power, dignity, and being.
In the incarnation, you have just the opposite. Instead of one essence and three persons, you have one person with two natures, and those two natures are the divine and the human. We have to be very careful not to confuse the divine and human natures of Jesus. If we do that, we end up with a deified human or a humanized deity, who in and of Himself is neither really human nor divine. The church has had to wrestle with that in past ages, which is why they’re very careful to distinguish between Christ’s two natures—the human and the divine.
So, when we say that Jesus is God, we don’t mean that the whole of Jesus is divine, because the human nature is not divine. But He has a divine nature, and that’s what we’re saying when we say Jesus is God—we’re saying that He is God incarnate, God united with a human nature.