Excerpted from Summa Theologiae 1.109, 114

The assault itself comes from the malice of the demons, who out of envy strive to impede man's progress; and because of their arrogance they assume a pretense of the divine power, by assigning chosen ministers to assaults on men, just as angels serve as ministers to God in chosen tasks pertaining to man's salvation. But the ordering of the assault itself is from God, who knows how to make use of the bad in an orderly way, by ordering it to good things. But on the side of angels, this charge, and the ordering of the charge, is referred back to God as its first author.

The Devil always tempts in order to do harm by driving men to sin. And in accordance with this, it is his proper task to tempt, for although man sometimes tempts in this way, he does this insofar as he acts as the Devil's agent. God, on the other hand, is said to tempt in order to know, so to speak in that he is said to know what he makes others know. Thus it is said in Deuteronomy 13.3, “The Lord your God is testing you to find out whether you love him.”

Original Latin

Ad tertium dicendum quod homo potest per seipsum ruere in peccatum, sed ad meritum proficere non potest nisi auxilio divino, quod homini exhibetur mediante ministerio Angelorum.

Respondeo dicendum quod, sicut ex supra dictis patet, si miraculum proprie accipiatur, Daemones miracula facere non possunt, nec aliqua creatura, sed solus Deus, quia miraculum proprie dicitur quod fit praeter ordinem totius naturae creatae, sub quo ordine continetur omnis virtus creaturae. Dicitur tamen quandoque miraculum large, quod excedit humanam facultatem et considerationem. Et sic Daemones possunt facere miracula, quae scilicet homines mirantur, inquantum eorum facultatem et cognitionem excedunt.

Nam et unus homo, inquantum facit aliquid quod est supra facultatem et cognitionem alterius, ducit alium in admirationem sui operis, ut quodammodo miraculum videatur operari.

Maxima autem perfectio, et per quam maxime Deo appropinquatur, est creaturarum fruentium Deo, sicut sunt sancti Angeli, qua perfectione Daemones privantur. Et ideo boni Angeli super malos praelationem habent, et per eos reguntur.