Prelest, that is, spiritual deception, readily tricks people who hold so much pride that they believe no one or nothing can ever deceive them. They think so highly of themselves, they will never question themselves, their experiences and their interpretation of those experiences, especially if those experiences can serve to reinforce the notion that they are somehow great and should be listened to by others. Prelest relies upon someone to not be willing to consider the possibility that they can be mistaken. Those who are unable to question themselves, and their beliefs, are easily deluded. Anything which would suggest they are in error they wave away with examination based upon confirmation bias.
On the other hand, those who are humble will be able to question themselves, questions which make it harder for them to be tricked. They will understand their own imperfections and how those imperfection impose limitations upon them and their ability to interpret and understand spiritual realities.
While much spiritual deception comes from the person who is deceived, that is, much of it is self-deception, it is clear that people can be deceived by others, and also, by spiritual powers (demonic powers) which want to have them deceived. It is important that those who, for one reason or another, deal with spiritual realities and the powers that be which can be found in them, be it through speculative theology or through mystical experience, to be careful when dealing with those spiritual realities. They should not let them draw them in. They should be cautious in accepted their experiences at face value. This is a warning consistently given by spiritual masters to novices, which is why we find this kind of advice represented by a large number of sayings coming from the desert fathers and mothers. We read from the early desert monastic community many warnings telling people not to take great concern with or interest in those spiritual experiences, and the potential beings that they might encounter, as often, what they encounter will not be what they seem to be. Indeed, they tell us not to be interested in meeting with our encountering spiritual entities, and if they come to us, presenting themselves, to us, we should dismiss them by embracing humility:
The devil appeared to one of the brothers disguised as an angel of light and said to him: “I am Gabriel and I was sent to you”, but he said to him: “Make sure you were not sent to another [person] for I am not worthy”, and [the devil] immediately became invisible. 
Those who are humble will question why some spirit or vision is given to them; they will think that they are not ready for them, and as such, will, with their humility, dismiss it in one fashion or another. Such humility, of course, should not turn into self-hatred, for that too leads to a great spiritual delusion, opening the person to other kinds of spiritual conflicts. Rather, humility should serve as a counter to vainglory and pride, and against those who would use vainglory or pride to trick them. A humble person will know that if there were any good associated with some spiritual experience they have, it is due to grace, not themselves, which is why they know they can and will continue to receive that grace even if they shrug off the spiritual experience itself. The experience is, in a way, a vessel for grace. The important thing is the grace itself. Even if an experience is true, a humble reaction to it is important in order to make sure it is not misconstrued. Christians must never become so attached to private revelation they let it distract them from the greater truths of the Christian faith. Sadly, we find many who are attracted to potential apparitions and locutions falling for this error as they end up considering the message of those apparitions and locutions as being more important than they are. And, due to their love for the supernatural, due to their attraction to apparitions, most of them tend not to question the veracity of the apparition, and so readily accept ones which are clearly false. Their spiritual life becomes so distorted that, in the end, even the good which could have been had by a true apparition becomes subverted and used as a tool for some evil. Thus, the desert fathers and mothers, who understood this danger, told their audience to never be so concerned with spiritual beings, with apparitions and locutions. They made it clear a Christian should be focused on practical concerns, on the basic elements of their spiritual progression which can be much more easily ascertained, instead of becoming so concerned with the supernatural they end up disengaging themselves from the norms of the Christian faith and the grace Christ shares with them:
The elders used to say: “Even if an angel appears to you, do not receive it, but humble yourself saying: ‘Living in sins, I am not worthy to see an angel.’” 
Again, the point is not to become self-hating, to loathe oneself, but rather, to make sure our focus remains properly centered, to make sure we do not become stuck as we are thanks to some attachment to a spiritual experience we might have had. Sadly, this warning, despite being repeated consistently by spiritual masters, is not being heeded by those who are more concerned with apparitions, such as Fatima, than they are in the basics of the Christian faith. They have come to believe their apparition(s) of choice are central to the Christian life, and they legalistically hold onto the messages they hear associated with the apparition, demanding others to do likewise or warn them that they risk eternal perdition for insulting God in that fashion. They act as belief in some secondary, private revelation, is more important than belief in public revelation, and as such, they end up falling for prelest. The ideal Christian life is one which is focused and centered upon Christ and what Christ presented to us through public revelation. We are to embrace the way of love, and through it, look to and help those around us:
Amma Eugenia said: “It is to our advantage to intercede and just to be with Jesus, for rich is everyone who is with Jesus, even though poor in bodily terms. For he who prefers the benefits on earth to spiritual ones will lose them both, while he who longs for heavenly benefits will obtain all the earthly ones too.” 
Many of those who encourage us to focus on apparitions often do so for the sake of material, that is, earthly benefits. There is big business around apparitions, and they are trying to find a way to make a profit through the beliefs and fears of others. Similarly, many find fame and notoriety through the proclamation of their supposed spiritual experiences, and as such, once again, seek a material gain, showing how far from spiritual they really are. The focus is so off base. This is why delusion is the end result. We must be cautious. We must keep our focus centered upon Christ and what Christ has already revealed to us. We should not pay much attention to those who want our attention, who want to use spiritual experiences and beliefs as a tool for their own benefit. And if we have some sort of experience ourselves, we should engage it with humility, realizing if it were true, the grace it imparts is enough, and if it is not, our focus on it will lead us to all kinds of delusion. We need to understand our lives are meant to be one of constant spiritual growth, something which becomes impossible if we end up focusing ourselves on any particular experience. We must let the experience move on, so that we can move on. That way, whether it is true or false, we will not let it get in the way of our life in Christ.
 John Wortley, trans., The Anonymous Sayings Of The Desert Fathers: A Select Edition And Complete English Translation (Cambridge: Cambridge University Press, 2013), 207 [N310/ 15.87].
 John Wortley, trans., The Anonymous Sayings Of The Desert Fathers, 207 [N311/ 15.88].
 John Wortley, trans., The Anonymous Sayings Of The Desert Fathers, 283[N 447].
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