All of the various forms of self-deception or prelest fall into two categories . . . The first kind often ends in insanity and suicide. The second, although it more rarely ends so tragically, is nonetheless just as ruinous.
The path to spiritual transfiguration, and indeed the search for truth on any level, meet with obstacles – not so often a direct opposition as a subtle, Satanic allurement – of which every Orthodox Christian should be aware.
The chief danger in undertaking ascetic endeavor lies in the possibility of becoming subject to self-deception or prelest. “All the forms of prelest,” says Bp. Ignaty,  “to which the athlete of prayer is subject arise from the fact that repentance has not been placed at the foundation of prayer that repentance has not been made the soul and aim of prayer. He who attempts to ascend to the wedding of the Son of God not in clean and bright wedding clothes,  which are made by repentance, but straightway in rags, in a state of self-deception and sinfulness, is cast forth into the outer darkness: into demonic prelest.”
Humility is the constant companion of sanctity; sanctity is unthinkable without it. “The humility with which St. Simeon the New Theologian recognizes his imperfection, and contritely repents his past sins and falls, serves as a guarantee that his mystical experience is completely free of the element of prelest and spiritual pride. In ascetical literature there are innumerable warnings to those jut beginning the monastic life not to succumb to false visions, not to be seduced, not to take an angel of darkness for an Angel of Light. Likewise we find in St. Simeon admonishments not to trust the unimaginable variety of knockings, voices, frightful apparitions, visions of perceptible light, fragrant odors, and so forth which come to the ascetic at prayer. . . Together with humility, the mystic is protected from the danger of falling into some kind of false mysticism of a secret tie with the Church. 
All of the various forms of self-deception or prelest fall into two categories and proceed, first, from defective activity of the mind, and second, from defective activity of the heart (feeling). “It is foolish pride to desire and strive to see spiritual visions with mind not purified of passions and not renewed and recreated by the right hand of the Holy Spirit, it is the same kind of pride and foolishness for the heart to desire and strive for the enjoyment of holy and Divine feelings, when it is still completely unfit for them” (Bp. Ignaty).
The first kind of prelest, owing to excitement of the mind and imagination, often ends in insanity and suicide; the second, which is called “fancy,” although it more rarely ends so tragically – because fancy, even though it leads the mind into the most frightful delusions, still does not cast it into delirium as in the first case – is nonetheless just as ruinous. The ascetic, striving to kindle in his heart love for God while neglecting repentance, exerts himself to attain a feeling of delight, of ecstasy, and as a result he attains precisely the opposite: “he enters into communion with Satan and becomes infected with hatred for the Holy Spirit.”
“Fancy,” in various degrees, is to be found everywhere: “Everyone who does not have a contrite spirit, who recognizes any kind of merit and worth in himself; everyone who does no hold unwaveringly the teaching of the Orthodox Church, but on some tradition or other has thought out his own arbitrary judgment or followed a non-Orthodox teaching – is in this state of prelest. The degree of prelest is determined by the degree of deviation and obstinacy in deviation” (Bp Ignaty).
In the fallen state, out of all feelings only one “can be utilized in invisible worship: sorrow for sins, for sinfulness, contrition of spirit . . . Sacrifice to God is a contrite spirit: a contrite and humbled heart God will not despise (Ps. 50:17)” (Bp. Ignaty).
Let us cite a characteristic instance of prelest arising from excitement of the mind and imagination, taken from the words of Bp. Ignaty. A monk visited him and said to him: “Father, pray for me, I sleep and eat much.” As he was saying this, Bp. Ignaty felt a heat issuing from him. In order to make clear the spiritual state of the monk, Bp. Ignaty asked him to instruct him in prayer. “And, horrors!” this monk began to teach him a “method of ecstatic, visionary prayer.” In what followed it became clear that the monk was completely unacquainted with the teaching of the Holy Fathers on prayer. “In the course of our conversation I said to him,” Bp. Ignaty further relates: “ ‘Look, Starets, you are going to be living in Petersburg; don’t by any means take a room on an upper floor; be sure to take one on the ground floor.’ ‘Why so?’ replied the monk. ‘Because,’ I replied, ‘if the thought ever occurs to angels suddenly to seize you and transport you from the ground floor and drop you, you will only injure yourself.’ ‘If you only knew,’ replied the monk, ‘how many times, when I have been standing at prayer, the vivid thought came to me that angels would carry me off and put me on Athos.’ It turned out that this schema-hieromonk wore chains, hardly ever slept, ate little food, and felt such heat in his body that he needed no warm clothing in winter. Towards the end of the conversation it entered my mind to take the following tack: I began to ask the monk, as a faster and ascetic, to try the method taught by the Holy Fathers, whereby the mind during prayer becomes free from every kind of fantasy, becomes entirely absorbed in attentiveness to the words of the prayer, is confined and held, as St. John of the Ladder expresses it (Step 28:17), in the words of the prayer, while at the same time the heart usually expresses its accord with the mind by means of the soul-saving feeling of sorrow for sins, as St. Mark the Ascetic said: “When the mind prays without distraction, the heart is contrite: A contrite and humbled heart God will not despise’ (226 Texts, Para 24; Philokalia, Vol 1). ‘When you have tried this for yourself,’ I told the monk, ‘inform me of the result of your experience, because it would be awkward for me, with the distracted life I lead, to undertake such an experiment.’
“The monk agreed. In a few days he came to me and said: ‘what have you done to me!’ ‘What is wrong?’ ‘Well, when I tried to pray with attentiveness, confining the mind in the words of the prayer, all my visions disappeared, and I can’t get them back any more.’ Conversing further with the monk, I did not see that boldness and self-reliance which were noticeable in him during our first meeting and which are usually to be noted in people who are in a state of self-deception, supposing themselves to be holy or to be progressing spiritually. The monk expressed a desire to hear my poor advice. When I advised him not to distinguish himself in appearance room others, since that leads to self-conceit, he took his chains off and handed them to me. In a month he came to me again and said that the heat in his body had ceased, that he was already needing warm clothing, and that he slept a great deal more. At the same time he said that on Mt. Athos many, even among those enjoying a reputation of holiness, use the method of prayer which he had used, and teach it to others as well” (Bp. Ignaty).
[Orthodox Word] Editor’s note: This example of prelest cited above, taken from the monastic life, should not lead one to suppose that it is a danger only to monks and ascetics: it has a powerful influence as well at very elementary levels of the spiritual life.
In the additional selection below from the same essay, Bp. Ignaty describes the deceptiveness of the IMITATION OF CHRIST by Thomas a Kempis, a Catholic religious manual still very popular in the West, although now considered “old fashioned” by Catholic modernists who have advanced to other forms of prelest.
“There reigns in this book breathes from its pages the unction of the evil spirit, flattering the reader, intoxicating him with the poison of untruth. . . The book conducts the reader directly to communion with God, without previous purification by repentance; that is why it finds a special sympathy among passionate people, those unacquainted with the path of repentance, not preserved from self-deception and prelest, not set on a proper foundation by the teaching of the Holy Fathers of the Orthodox Church. The book produces a strong effect on the blood and nerves, excites them, which is why it is especially pleasing to people enslaved by sensuality: with this book they can enjoy themselves without renouncing the delights of sensuality. Self-conceit, refined sensuality, and vanity are set forth by the book in place of the action of Divine grace. . . From it carnal people enter into rapture from a delight and intoxication attained without crucifixion of the flesh with its passions and desires, with flattery of their fallen state.”
 All citations from On Prelest in Vol 1 of the complete Works of Bishop Ignaty, St. Petersburg, 165, pp 132-148.
 In antiquity, a king would send a special court dress to those invited to his banquet. In the parable of the Saviour, the wedding clothes symbolize the grace of the Holy Spirit sent by God to the ascetic.
 Archim. Prof. Cyprian. “The Spiritual Forerunners of (St) Gregory Palamas,” in Theological Thought, 1942, p 113.