Hieromartyr Bishop Arsenius (Zhadanovsky). "Spiritual Diary".
Not only the people indulging in vices and passions are in the enemy's nets, are the accomplices of the enemy of our salvation, but those who set themselves the task to live piously are not left alone by the same enemy; he is trying to lure these Christians in his nets too, but rather based on their piety - that is, he involves them into so-called prelest, or spiritual delusion. Prelest is a perverted, wrong disposition, supposedly high, spiritual, but in fact false, deceptive, fake, sham, - this is the internal state of our hearts, of which the Apostle Paul says that one can only have a form of godliness, but be denying the power thereof.
Yet everyone in his feeble, sinful nature is more or less susceptible to this delusion. You thought that you said well, did well - that is delusion. You were dreaming about your talents, abilities - again delusion. Accepted praise, took a delight in it - and that is delusion too. The Holy Fathers distinguish two kinds of spiritual delusion in its extreme development. The first kind is when a person begins to imagine that he saw the Lord, the Mother of God, Angels, spirits, and considers himself worthy of such visions. The second kind of prelest is when a person has a high opinion of himself, thinking that he lives a high spiritual life, that he fasts a lot, he is a clairvoyant person, a miracle worker, that he is able to perform great ascetic feats, that he is destined to teach and guide people. This kind of delusion is called “conceit”, because in this case the person conceited opinion about himself.