The Catechism of the Summa - The Second Part:XII. OF ORIGINAL SIN, AND OF ITS CONSEQUENCES, OR OF THE WOUNDING OF HUMAN NATURE (C)

(C)   In spite of all these causes of sin in man which come either from the sin of Adam, or from the personal sins of man, may we yet say that man is free in his moral acts, and that he is never necessitated to commit sin?

Yes, in spite of all these causes of sin in man which come either from the first sin of the first man, or from the personal sins of man, we are bound to say that he is still free in his moral acts, and that he is never necessitated to commit sin.
 
What would be necessary for man to cease to be free in his acts considering all these consequences of sin?

It would be necessary for them to affect man in such a way as to make him lose his reason (LXXVII. 7).
 
Unless man then loses his reason he always remains free in his acts in such wise that it depends upon him whether he sin?

Yes, unless man loses his reason his acts always remain free in such wise that it depends upon him whether he sin.
 
Can this liberty, however, become less perfect and less virile on account of the consequences of sin, even to the extent that when man relapses into sin he is less culpable?

Yes, man's liberty becomes less perfect and less virile owing to the effects of sin, so much so that when he relapses into sin he is less culpable, unless his personal sins are themselves in part cause of this abatement of his perfect liberty (LXXVII. 6).

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