St. Theophan the Recluse, "Letters".
Beware of spiritual deception! You really have to. But avoid the cornerstone spiritual deception and the others will not take place. The cornerstone spiritual deception is to think and even to feel (which is much worse) that I am something meanwhile I am nothing. I call that ‘self-value’. Do pay great attention to it, may this ‘self-value’ became your worst foe. Do not let this foe settle in, otherways, it will lead you to perish. That is an Apostolic lesson: For if a man think himself to be something, when he is nothing, he deceiveth himself (Gal 6:3).