Angels and men, constituted in a state of nature, may sin; and if any creature cannot sin it is due to the gift of grace, not to their nature; for to sin is simply to decline from the rectitude of action which is incumbent, whether in things natural, artificial or moral. The Will of God only cannot decline because it is the rule of His Acts, and not ordered to a superior end; and the will of every creature has rectitude of action in proportion as it is regulated by the Divine Will, to which the final end belongs. Thus, according to the conditions of nature, sin may exist in every creature.
It may exist, moreover, in the said creature after a twofold manner, viz. as cause, or by affection. According to the former, every sin may be found in the devils, because they incur the guilt of every sin which they induce men to commit; but according to affection, they are liable to those sins only to which a spiritual nature can be attached, or which belong to such. These sins consist in the disorder of their affection to spiritual good, through not remaining subject to the rule of the superior. Hence the first sin of the angels can only be pride, which is crossed by the orders of a superior; afterwards there may also be envy, inasmuch as they are grieved by the good of men, or esteem the good of others as an impediment to their own.
The angels are said to have sinned through desiring to be like God; which may be understood in two senses: either as a desire of equality, or of similitude. According to the former, the angels could not desire what they knew by nature to be impossible; it is, moreover, contrary to the natural desire of self-preservation for any creature to wish to change its being. In the latter sense there are two kinds of assimilation to God: one is the end for which a thing is created; therefore to desire such a likeness is good, provided it be in due order. But to desire likeness to God for which they were not created would be sinful, and in this sense the devil desired to be as God, coveting that which he could attain by his natural strength as the final end of his Beatitude, and averting his desire from the supernatural Beatitude which is by the grace of God. In other words, he desired to possess the Divine Similitude by his own power, not by the Divine assistance.
The devils are not bad in nature; for every nature being ordered to some good, according to the common conception of goodness, theirs cannot tend to evil. The intellectual nature, indeed, bears relation to Universal Good, which it is capable of apprehending, and which is the object of will; hence it is impossible that they could have an inherent inclination to evil: for although some evil may be joined by accident to a particular good - as fire, although good by nature, may happen to destroy something - no evil can be joined to the Universal Good.
Nor could the angels be bad by default of their own will, at the first moment of creation. Besides being contrary to the authority of Ezekiel (28:13), "Thou wast in the delights of Paradise," it is impossible that they should have sinned in the first instant, because God, Who created them, is not a defective Cause, nor a Cause of sin. For an operation may be defective through its relation to the agent, as when a child, born lame through hereditary weakness, limps from the beginning; but this could not be the case with the angels, created directly by God, in Whom there is no defect.
According to the more probable opinion of the saints, the devils sinned immediately after the first moment of their creation; thus, if created in grace, as was said above, some angels obtained Beatitude by one meritorious act, while others placed an impediment by the use of their free will; should it be admitted, however, that they were not created in grace, or that they did not exercise freewill at the first moment of their creation, there may have been some delay between their creation and fall.
As regards sin, we have to consider the proneness to sin and the motive for sinning. According to the opinion of Saint Gregory, the first angel that fell was of superior grade, the motive being pride, which arises from excellence; for the sin of the angels cannot be ascribed to any proneness to evil, but to free will alone. It is also evident that the sin of the first angel was the cause of the fall of others, not as obliging, but as inducing them to sin, from Matthew 25:41: "Go, ye cursed," etc., which shows that the other demons were subject to the supreme one; and that, by the order of Divine Justice, those who have been guilty of yielding to temptation are subjected in punishment to the evil power which has conquered them (2 Peter 2:19).
Nor do the angels require time for deliberation, choice and consent, as we do. With them all this is the work of an instant; for since they are moved by one single faculty, in whatever direction, there is nothing to keep them back.
The angels who remained steadfast were more in number than those who fell; for sin is contrary to natural inclination, and nature obtains its effect always or generally; and if such is not found to be the case with men it is because they are liable to follow the sensible goods, which are more known to them; a reason which does not exist in the angels, whose nature is purely intellectual.
If the first angel who fell was one of inferior rank, they did not fall from all the Orders; but the contrary opinion is the more probable, since men are held to be assumed into all the Orders, to repair the losses of the angelic hierarchy.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni