Objection 1. It would seem that men are not assailed by the demons. For angels are sent by God to guard man. But demons are not sent by God: for the demons' intention is the loss of souls; whereas God's is the salvation of souls. Therefore demons are not deputed to assail man.
Objection 2. 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.
Objection 3. 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.
On the contrary, The Apostle says (Ephesians 6:12): "Our wrestling is not against flesh and blood; but against Principalities and Powers, against the rulers of the world of this darkness, against the spirits of wickedness in the high places."
I answer that, Two things may be considered in the assault of the demons--the assault itself, and the ordering thereof. The assault itself is due to the malice of the demons, who through envy endeavor to hinder man's progress; and through pride usurp a semblance of Divine power, by deputing certain ministers to assail man, as the angels of God in their various offices minister to man's salvation. But the ordering of the assault is from God, Who knows how to make orderly use of evil by ordering it to good. On the other hand, in regard to the angels, both their guardianship and the ordering thereof are to be referred to God as their first author.
Reply to Objection 1. 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 1 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; whereas they are sent by God on account of His justice.
Reply to Objection 2. In order that the conditions of the fight be not unequal, there is as regards man the promised recompense, to be gained principally through the grace of God, secondarily through the guardianship of the angels. Wherefore (2 Kings 6:16), Eliseus said to his servant: "Fear not, for there are more with us than with them."
Reply to Objection 3. The assault of the flesh and the world would suffice for the exercise of human weakness: but it does not suffice for the demon's malice, which makes use of both the above in assailing men. But by the Divine ordinance this tends to the glory of the elect.
Source: Summa Theologica (New Advent)