15 But sin also is called spirit, as I have already said; only in another and opposite sense, as when it is said, The spirit of whoredom caused them to err. The name “spirit” is given also to the unclean spirit, the devil; but with the addition of, “the unclean;” for to each is joined its distinguishing name, to mark its proper nature. If the Scripture speak of the soul of man, it says the spirit with the addition, of the man; if it mean the wind, it says, spirit of storm; if sin, it says, spirit of whoredom; if the devil, it says, an unclean spirit: that we may know which particular thing is spoken of, and you may not suppose that it means the Holy Ghost; God forbid!
For this name of spirit is common to many things; and every thing which has not a solid body is in a general way called spirit. Since, therefore, the devils have not such bodies, they are called spirits: but there is a great difference; for the unclean devil, when he comes upon a man's soul (may the Lord deliver from him every soul of those who hear me, and of those who are not present), he comes like a wolf upon a sheep, ravening for blood, and ready to devour. His coming is most fierce; the sense of it most oppressive; the mind becomes darkened; his attack is an injustice also, and so is his usurpation of another's possession.
For he makes forcible use of another's body, and another's instruments, as if they were his own; he throws down him who stands upright (for he is akin to him who fell from heaven); he twists the tongue and distorts the lips; foam comes instead of words; the man is filled with darkness; his eye is open, yet the soul sees not through it; and the miserable man gasps convulsively at the point of death. The devils are verily foes of men, using them foully and mercilessly.
Source: Catechetical Lectures (New Advent)