The grace-nature distinction provides a subjectivity that can receive the offer of grace and guarantees the gratuity. The essence of man is different than the nature. It includes the actual finality. For Von Balthasar, the essence is like Rahner's quiddity. Because God is creator and we are creatures, the dynamics between freedom and necessity are different than any other we know in this creation.
Grace is never intrinsically necessary.
It is always freely presented. A graceless world is possible, but unknowable, but unnecessary to assure the gratuity of grace. We know too little about it for it to be of any use. Because Von Balthasar associates nature with the cosmic order, he denies that the desire to see God is natural. But he does affirm that the desire to see God is essential to humanity.