Be concentrated without self-display, withdrawn into your heart. For the demons fear concentration as thieves fear dogs.

St. John Climacus

Whatever the soul may think fit to do itself, whatever care and pains it may take, relying only upon its own power, and thinking to be able to effect a perfect success by itself, without the co-operation of the Spirit, it is greatly mistaken. It is of no use for the heavenly places; it is of no use for the kingdom – that soul, which supposes that it can achieve perfect purity of itself, and by itself alone, without the Spirit. Unless the man who is under the influence of the passions will come to God, denying the world, and will believe with patience and hope to receive a good thing foreign to his own nature, namely the power of the Holy Spirit, and unless the Lord shall drop upon the soul from on high the life of the Godhead, such a man will never experience true life, will never recover from the drunkenness of materialism; the enlightenment of the Spirit will never shine in that benighted soul, or kindle in it a holy daytime; it will never awake out of that deepest sleep of ignorance, and so come to know God of a truth through God’s power and the efficacy of grace.

St. Macarius the Great

Sinful thoughts continually disturb a man. But if he does not cooperate with them, then he is not guilty of them. 

St. Ambrose of Optina

In the words of the psalmist, "As you lie in bed, repent of what you say in your heart" (Ps. 4:4 LXX), that is, repent in the stillness of the night, remembering the lapses that occurred in the confusion of the day and disciplining yourself in hymns and spiritual songs (cf. Col. 3:16) – in other words, teaching yourself to persist in prayer and psalmody through attentive meditation on what you read. For the practice of the moral virtues is effectuated by meditating on what has happened during the day, so that during the stillness of the night we can become aware of the sins we have committed and can grieve over them.

St. Peter of Damascus

The proud man thinks he can comprehend everything with his mind, but God does not grant this.  But we know the Lord:  He has made Himself manifest to us in the Holy Spirit, and the soul knows and is joyful and at ease, and in this lies our hallowed life.

St. Silouan the Athonite