Excerpt from The Life of St. Antony:

'First, therefore, we must know this: that the demons have not been created like what we mean when we call them by that name for God made nothing evil, but even they have been made good. Having fallen, however, from the heavenly wisdom, since then they have been grovelling on earth. On the one hand they deceived the Greeks with their displays, while out of envy of us Christians they move all things in their desire to hinder us from entry into the heavens; in order that we should not ascend up thither from whence they fell. Thus there is need of much prayer and of discipline, that when a man has received through the Spirit the gift of discerning spirits, he may have power to recognise their characteristics: which of them are less and which more evil; of what nature is the special pursuit of each, and how each of them is overthrown and cast out. For their villainies and the changes in their plots are many. The blessed Apostle and his followers knew such things when they said, "for we are not ignorant of his devices ;" and we, from the temptations we have suffered at their hands, ought to correct one another under them. Wherefore I, having had proof of them, speak as to children.