The Enchiridion

The former immortality man lost through the exercise of his free-will; the latter he shall obtain through grace, whereas, if he had not sinned, he should have obtained it by desert. Even in that case, however, there could have been no merit without grace; because, although the mere exercise of man's free-will was sufficient to bring in sin, his free-will would not have sufficed for his maintenance in righteousness, unless God had assisted it by imparting a portion of His unchangeable goodness.

Just as it is in man's power to die whenever he will (for, not to speak of other means, any one can put an end to himself by simple abstinence from food), but the mere will cannot preserve life in the absence of food and the other means of life; so man in paradise was able of his mere will, simply by abandoning righteousness, to destroy himself; but to have maintained a life of righteousness would have been too much for his will, unless it had been sustained by the Creator's power.

After the fall, however, a more abundant exercise of God's mercy was required, because the will itself had to be freed from the bondage in which it was held by sin and death. And the will owes its freedom in no degree to itself, but solely to the grace of God which comes by faith in Jesus Christ; so that the very will, through which we accept all the other gifts of God which lead us on to His eternal gift, is itself prepared of the Lord, as the Scripture says.