St. Theophan the Recluse: How did our ascetics, fathers, and teachers warm the spirit of prayer inwardly...

How did our ascetics, fathers, and teachers warm the spirit of prayer inwardly, and establish themselves firmly in prayer? Their great object was to make the heart burn unceasingly towards the Lord alone. 

God claims the heart because within it lies the source of life. 

Where the heart is, there is consciousness, attention, mind; there is the whole soul. When the heart is in God, then the whole soul is in God, and man remains in unceasing worship of Him in spirit and in truth.

St. Theophan the Recluse