The enemy is within ourselves. An invisible war is taking place within us. If interior evil is defeated, then the external, weaker, foe will surrender.
St. John Cassian
 
Those who strive to live a spiritual life experience the most subtle and difficult of spiritual wars: a battle throughout each moment of their lives, with their thoughts.
St. John of Kronstadt
 
There exists within us greater mental warfare than sensual warfare.
Venerable St. Philotheus of Sinai
 
The central point in the matter of internal warfare lies in the following: go down attentively into your heart. Stand there before the Lord and do not permit anything sinful to enter in. In this lies the fullness of interior warfare.
St. Theophan the Recluse