I prefer believer. Although brother, son of God, and saint are fine, too. Most people just use my first name.
We must understand, believing is the de facto reality for man: they were created to believe. Unbelief is the opposite of belief. You will not find "non-believer" in the scriptures: man either believes God or resists Him.
There are only two sons in creation: one is Adam born to sin the other is Christ, a life-giving Spirit. Through rebellion, man may resist God. Such a man is an unbeliever. If man let's the Spirit of God work in His life he becomes a believer.
Furthermore, belief is active, not passive. The fruit of belief or unbelief is always seen in the activity of a man.
This is merely an outline but it captures the gist.