After the devil and his demons had brought about man’s exile from paradise and from God by making him transgress, they found they could inwardly derange–to a greater or lesser extent–anyone’s reason whenever they wanted to. The only defense against this is the ceaseless mindfulness of God, for if such mindfulness is stamped on the heart through the power of the cross it will render our thought steadfast and unshakeable.
This is a state to which the spiritual contest of every Christian who enters the arena of Christ’s faith should lead, if he is not to struggle in vain. For it is to achieve this state that God’s athlete embraces all the various forms of ascetic practice.
~St Symeon the New Theologian
Prayerful Life 