Thomas Aquinas

“ Wherefore, an angel's mind is not directed to the means, except as they stand under the order to the end. Consequently, from their very nature, they can have no inordinateness in respect of the means, unless at the same time they have an inordinateness in respect of the end, and this is a mortal sin. Now good angels are not moved to the means, except in subordination to the due end which is God: wherefore all their acts are acts of charity, so that no venial sin can be in them. On the other hand, wicked angels are moved to nothing except in subordination to the end which is their sin of pride. ”

“ It seems that a good or wicked angel can sin venially. Because man agrees with the angels in the higher part of his soul which is called the mind, according to Gregory, who says (Hom. xxix in Evang.) that "man understands in common with the angels." But man can commit a venial sin in the higher part of his soul. Therefore an angel can commit a venial sin also. ”

“ Therefore every act of concupiscence in an unbeliever, even without his consent, is a mortal sin, because he acts against his duty.
On the contrary, It is stated in Acts 10:34 that "God is not a respecter of persons." Therefore he does not impute to one unto condemnation, what He does not impute to another. But he does not impute first movements to believers, unto condemnation. Neither therefore does He impute them to unbelievers. ”

“ Although those who are withdrawn from the care of temporal things, sin venially sometimes, yet they commit but slight venial sins, and in most cases they are cleansed by the fervor of charity: wherefore they do not build up venial sins, because these do not remain long in them. But the venial sins of those who are busy about earthly remain longer, because they are unable to have such frequent recourse to the fervor of charity in order to remove them. ”

“ Others say that wood, hay, stubble designate good works, which are indeed built upon the spiritual edifice, but are mixed with venial sins: as, when a man is charged with the care of a family, which is a good thing, excessive love of his wife or of his children or of his possessions insinuates itself into his life, under God however, so that, to wit, for the sake of these things he would be unwilling to do anything in opposition to God. ”

“ We must therefore say that the very venial sins that insinuate themselves into those who have a care for earthly things, are designated by wood, hay, and stubble. For just as these are stored in a house, without belonging to the substance of the house, and can be burnt, while the house is saved, so also venial sins are multiplied in a man, while the spiritual edifice remains, and for them, man suffers fire, either of temporal trials in this life, or of purgatory after this life, and yet he is saved for ever. ”

“ Now venial sin is a hindrance to actual comeliness, but not to habitual comeliness, because it neither destroys nor diminishes the habit of charity and of the other virtues, as we shall show further on (SS, Q [24] , A [10] ; Q [133] , A [1] , ad 2) , but only hinders their acts. On the other hand a stain denotes something permanent in the thing stained, wherefore it seems in the nature of a loss of habitual rather than of actual comeliness. ”

“ But sometimes the man who commits a venial sin, will not be saved, even by fire, e.g. when a man dies in mortal sin to which venial sins are attached. Therefore, venial sins are unsuitably designated by wood, hay, and stubble. ”

“ Further, He that can do more can do less. But an angel could love a created good more than God, and he did, by sinning mortally. Therefore he could also love a creature less than God inordinately, by sinning venially. ”

“ Therefore they sin mortally in everything that they do of their own will. This does not apply to the appetite for the natural good, which appetite we have stated to be in them (FP, Q [63] , A [4] ; Q [64] , A [2] , ad 5) .
Reply to Objection 1: Man does indeed agree with the angels in the mind or intellect, but he differs in his mode of understanding, as stated above. ”