The Enchiridion

Now it was expedient that man should be at first so created, as to have it in his power both to will what was right and to will what was wrong; not without reward if he willed the former, and not without punishment if he willed the latter. But in the future life it shall not be in his power to will evil; and yet this will constitute no restriction on the freedom of his will. On the contrary, his will shall be much freer when it shall be wholly impossible for him to be the slave of sin.

We should never think of blaming the will, or saying that it was no will, or that it was not to be called free, when we so desire happiness, that not only do we shrink from misery, but find it utterly impossible to do otherwise. As, then, the soul even now finds it impossible to desire unhappiness, so in future it shall be wholly impossible for it to desire sin. But God's arrangement was not to be broken, according to which He willed to show how good is a rational being who is able even to refrain from sin, and yet how much better is one who cannot sin at all; just as that was an inferior sort of immortality, and yet it was immortality, when it was possible for man to avoid death, although there is reserved for the future a more perfect immortality, when it shall be impossible for man to die.