Thomas Aquinas

“ The flesh and the world are said to tempt as the instruments or matter of temptations; inasmuch as one can know what sort of man someone is, according as he follows or resists the desires of the flesh, and according as he despises worldly advantages and adversity: of which things the devil also makes use in tempting. ”

“ The wicked angels assail men in two ways. Firstly by instigating them to sin; and thus they are not sent by God to assail us, but are sometimes permitted to do so according to God's just judgments. But sometimes their assault is a punishment to man: and thus they are sent by God; as the lying spirit was sent to punish Achab, King of Israel, as is related in 3 Kings 22:20. For punishment is referred to God as its first author. Nevertheless the demons who are sent to punish, do so with an intention other than that for which they are sent; for they punish from hatred or envy ”

“ The demons know what happens outwardly among men; but the inward disposition of man God alone knows, Who is the "weigher of spirits" (Prov. 16:2) . It is this disposition that makes man more prone to one vice than to another: hence the devil tempts, in order to explore this inward disposition of man, so that he may tempt him to that vice to which he is most prone. ”

“ It would seem that a demon who is overcome by a man, is not for that reason hindered from any further assault. For Christ overcame the tempter most effectively. Yet afterwards the demon assailed Him by instigating the Jews to kill Him. Therefore it is not true that the devil when conquered ceases his assaults. ”

“ Further, it is not a fair fight, for the weak to be set against the strong, and the ignorant against the astute. But men are weak and ignorant, whereas the demons are strong and astute. It is not therefore to be permitted by God, the author of all justice, that men should be assailed by demons. ”

“ And thus demons can work miracles, that is, things which rouse man's astonishment, by reason of their being beyond his power and outside his sphere of knowledge. For even a man by doing what is beyond the power and knowledge of another, leads him to marvel at what he has done, so that in a way he seems to that man to have worked a miracle.
It is to be noted, however, that although these works of demons which appear marvelous to us are not real miracles, they are sometimes nevertheless something real. Thus the magicians of Pharaoh by the demons' power produced real serpents and frogs. ”

“ Man can of his own accord fall into sin: but he cannot advance in merit without the Divine assistance, which is borne to man by the ministry of the angels. For this reason the angels take part in all our good works: whereas all our sins are not due to the demons' instigation. Nevertheless there is no kind of sin which is not sometimes due to the demons' suggestion.
Art. 4 - Whether demons can lead men astray by means of real miracles? Objection 1: It would seem that the demons cannot lead men astray by means of real miracles. ”

“ On the contrary, those transformations which cannot be produced by the power of nature, cannot in reality be effected by the operation of the demons; for instance, that the human body be changed into the body of a beast, or that the body of a dead man return to life. And if at times something of this sort seems to be effected by the operation of demons, it is not real but a mere semblance of reality. ”

“ Further, the assaults of the flesh and the world are enough for man's exercise. But God permits His elect to be assailed that they may be exercised. Therefore there is no need for them to be assailed by the demons. ”

“ But sometimes another end, either good or bad, is sought to be acquired through that knowledge; a good end, when, for instance, one desires to know of someone, what sort of a man he is as to knowledge, or virtue, with a view to his promotion ”

“ As Augustine says in the same place, the works of Antichrist may be called lying wonders, "either because he will deceive men's senses by means of phantoms, so that he will not really do what he will seem to do ”

“ The other reason is taken from the astuteness of the devil. As to this, Ambrose says on Lk. 4:13: "The devil is afraid of persisting, because he shrinks from frequent defeat." That the devil does nevertheless sometimes return to the assault, is apparent from Mat. ”