Augustine here argues that all sin is voluntary. Yet this does not limit the extent of sin as narrowly as one would suppose. This remark is taken from his Retractions 1, 13 concerning a remark he made in his book Of True Religion.
Sin is so much voluntary evil, that there would be no such thing as sin unless it were voluntary. . . . Sins which are not unjustifiably said to be non-voluntary because they are committed in ignorance or under compulsion cannot be said to be committed entirely involuntarily. He who sins in ignorance uses his will to some extent, for he thinks he should do what in fact ought not to be done. He who does not the things that he would because the flesh lusteth against the spirit, may be unwilling but he lusts all the same, and thereby does not the things he would. If he is overcome he voluntarily consents to lust, and thereby does what he wishes, being free from righteousness and the servant of sin. What we call original sin in infants, who have no yet the use of free choice, may not absurdly also be called voluntary, because it originated in man’s first evil will and has become in a manner hereditary.*
*Augustine, Earlier Writings, trans. John H. S. Burleigh (Philadelphia: Westminister Press, 1953), 219.