John Gerstner, Theology for Everyman, Moody, 1965, Chapter 10.
God has no obligation to sinful men except to condemn. He may or may not, as His wisdom dictates, exercise mercy upon them. But mercy is not something which God must offer anybody. He offered no mercy to the angels when they sinned. And He says with respect to fallen human creatures: “I will have mercy on whom I will have mercy” (Rom. 9:18). He strongly insists that mercy is optional with Him and a matter of His sovereign pleasure alone.
John Gerstner, Theology for Everyman, Moody, 1965, Chapter 10.