Continually take careful note of your inner intention: watch carefully which way it inclines, and discover whether it is for God and for the sake of goodness itself and the benefit of your soul that you practice stillness or psalmodize or read or pray or cultivate some virtue. Otherwise you may unknowingly be ensnared and prove to be an ascetic in outward appearance alone while in your manner of life and inner intention you are wanting to impress men, and not to conform to God.
For the devil’s traps are many, and he persistently and secretly watches the bias of our intention, without most of us being aware of it, striving imperceptibly to corrupt our labor so that what we do is not done in accordance with God’s will. But even if he attacks and assaults you relentlessly and shamelessly, and even if he distracts the bias of your will and makes it waver in spite of your efforts to prevent it, you will not often be caught out by him so long as you keep yourself steadfastly intent on God.
…There is, however, one passion–self-esteem–that does not permit [one] to grow in virtue, so that though he engages in ascetic labors in the end he remains barren. For whether you are a beginner, or midway along the spiritual path, or have attained the stage of perfection, self-esteem always tries to insinuate itself, and it nullifies your efforts to live a holy life, so that you waste your time in listlessness and day-dreaming.
~St Gregory of Sinai (Philokalia vol.4, pp.271-272)
Prayerful Life 