St. John of the Cross

Dark Night of the Soul (2.2:3)...

"...the devil causes many to believe in vain visions and false prophecies; and strives to make them presume that God and the saints are speaking with them; and they often trust their own fancy. And the devil is also accustomed, in this state, to fill them with presumption and pride, so that they become attracted by vanity and arrogance, and allow themselves to be seen engaging in outward acts which appear holy, such as raptures and other manifestations. Thus they become bold with God, and lose holy fear, which is the key and the custodian of all the virtues; and in some of these souls so many are the falsehoods and deceits which tend to multiply, and so inveterate do they grow, that it is very doubtful if such souls will return to the pure road of virtue and true spirituality."