The demons fight against men through their own malice, and being envious of man's advancement they instigate him to sin; but the order of the combat is from God, Whose Wisdom can utilize evil by overruling it for good. Hence the combat is due to their malice, while the order is from God Himself.
To tempt properly signifies to make experiment, so that something may be known about the person tempted. Temptations are therefore multiplied for the sake of the end, being sometimes permitted for the sake of the knowledge only, sometimes for an ulterior end, whether good or evil; but to tempt in order to injure belongs to the devil only. Man tempts both for good and evil; God for good only, in order that we may make progress.
The world and the flesh are also said to tempt materially and instrumentally; because a man's character is known according as he follows after, or resists the concupiscence of the flesh, or by the manner in which he accepts the prosperity or adversity through which the devil takes occasion to tempt him.
Every sin proceeds indirectly from the temptation of the devil, inasmuch as Satan was the cause of the first sin, from which a proneness to sin has followed; but the devil is not directly the cause of every sin, because man possesses free will, and sensitive appetite which the will ought to restrain; therefore we must conclude that man can teach himself evil, but cannot merit a supernatural reward without the help of God, which is given to him through the ministry of the angels.
Miracles, properly so called, cannot be worked by the devil, because they exceed the order and virtue of all created nature; but if the term be used broadly for things which surpass our faculties and knowledge, they may do so; but such, though real phenomena, are not real miracles; for the devils can produce whatever effects are possible through the powers of Nature, by means of seeds which are found in the elements of the world, but nothing more.
According to some, a demon once vanquished ceases absolutely to tempt; but it is more probable that he ceases only for a time; as we read that the devil left Christ for a time. But it is evident that the devil must shrink from defeat on account of his pride; and the same may happen also through the mercy of God in consideration of our infirmity. That the devils sometimes return we see from Matthew 12 where it is said: "I will return to my house whence I went out."
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni