John Cassian, The Institutes XI.2: How vainglory attacks a monk not only on his carnal, but also on his spiritual side

For not only does this, like the rest of his faults, attack a monk on his carnal side, but on his spiritual side as well, insinuating itself by craft and guile into his mind: so that those who cannot be deceived by carnal vices are more grievously wounded through their spiritual proficiency; and it is so much the worse to fight against, as it is harder to guard against. For the attack of all other vices is more open and straightforward, and in the case of each of them, when he who stirs them up is met by a determined refusal, he will go away the weaker for it, and the adversary who has been beaten will on the next occasion attack his victim with less vigour. But this malady when it has attacked the mind by means of carnal pride, and has been repulsed by the shield of reply, again, like some wickedness that takes many shapes, changes its former guise and character, and under the appearance of the virtues tries to strike down and destroy its conqueror.