St. Tikhon of Zadonsk.
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"Allow the Spirit of God to dwell within you; then in His love He will come and make a habitation with you; He will reside in you and live in you. If your heart is pure you will see Him and He will sow in you the good seed of reflection upon His actions and wonder at His majesty. This will happen if you take the trouble to weed out from your soul the undergrowth of desires, along with the thorns and tares of bad habits."
Evagrius of Pontus
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"This is the spiritual passion [i.e., the Love of God] with which the apostles and the martyrs were inebriated. With it they first travelled the world over, toiling and being reviled, while the second, although their members were severed, and although they shed their blood like water and suffered the most dreadful torments, yet they did not grow faint-hearted but endured courageously, and bring truly wise, were thought fools. Still others wandered in mountains and caves and dens of the earth, and amid disorder they were most well-ordered."
Saint Isaac the Syrian
[The Ascetical Homilies of Saint Isaac the Syrian]