John Cassian, The Institutes XI.9: That vainglory is the more dangerous through being mixed up with virtues

Lastly, other passions which are entirely different from the virtues which are their opposites, and which attack us openly and as it were in broad daylight, are more easily overcome and guarded against: but this being interwoven with our virtues and entangled in the battle, fighting as it were under cover of the darkness of night, deceives the more dangerously those who are off their guard and not on the lookout.