It would seem that the ingratitude, whereby a subsequent sin causes the return of sins previously forgiven, is a special sin. For, the giving of thanks belongs to counter passion, which is a necessary condition of justice. But justice is a special virtue. Therefore this ingratitude is a special sin. Thanksgiving is a special virtue. But ingratitude is opposed to thanksgiving. Therefore ingratitude is a special sin.
Thomas AquinasCommemoration of Gilbert of Sempringham, Founder of the Gilbertine Order, 1189 Some there are who presume so far on their wits that they think themselves capable of measuring the whole nature of things by their intellect, in that they esteem all things true which they see, and false which they see not. Accordingly, in order that man's mind might be freed from this presumption, and seek the truth humbly, it was necessary that certain things far surpassing his intellect should be proposed to man by God.
Thomas Aquinas