Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Works of John Cassian.: Chapter XIX. The answer how far an agreement exists among devils about the attack and its changes.

Chapter XIX.

The answer how far an agreement exists among devils about the attack and its changes.

Serenus: It is a true assertion that there is no lasting concord among bad men, and that perfect harmony cannot exist even in regard to those particular faults which have attractions for them all in common. For, as you have said, it can never be that system and discipline are preserved among undisciplined things. But in some matters, where community of interests, and necessity enforces it, or participation in some gain recommends it, they must arrange for some agreement for the time being. And we see very clearly that this is so in the case of this war of spiritual wickedness; so that not only do they observe times and changes among themselves, but actually are known specially to occupy some particular spots and to haunt them persistently: for since they must make their attacks through certain fixed temptations and well defined sins, and at particular times, we clearly infer from this that no one can at one and at the same time be deluded by the emptiness of vainglory and inflamed by the lust of fornication, nor at one and the same time be puffed up by the outrageous haughtiness of spiritual pride, and subject to the humiliation of carnal gluttony. Nor can anyone be overcome by silly giggling and laughter and at the same time be excited by the stings of anger, or at any rate filled with the pains of gnawing grief: but all the spirits must one by one advance to attack the soul, in such a way that when one has been vanquished and retreated, he must make way for another spirit to attack it still more vehemently, or if he has come forth victorious, he will none the less hand it over to be deceived by another.