Selected quotes from St. Simeon the New Theologian:

" 'Can a man take fire into his bosom, and his clothes not be burned?' (Prov. 6:27) says the wise Solomon. And I say: can he, who has in his heart the Divine fire of the Holy Spirit burning naked, not be set on fire, not shine and glitter and not take on the radiance of the Deity in the degree of is purification and penetration by fire? For penetration by fire follows upon purification of the heart, and again purification of the heart follows upon penetration by fire, that is, inasmuch as the heart is purified, so it receives Divine grace, and again inasmuch as it receives grace, so it is purified. When this is completed (that is, purification of heart and acquisition of grace have attained their fullness and perfection), through grace a man becomes wholly a god."

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"As a blacksmith can do nothing without the help of fire, however skilled he may be in wielding his tools, so a man should do everything he can on his side to purify his heart, using virtues as tools for this purpose; but without fire of the Spirit, everything he does will remain inactive and useless for his aim, for by itself what he does is powerless to cleanse the soul of its dirt and foulness."

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"He who has acquired purity of heart has conquered fear. But a man who is still in process of purification, at times is conquered by fear, and at times conquers it. A man, however, who does not strive at all after purity, is either for ever sunk into a state of insensibility and, being a friend of passions and demons and filled with vanity and conceit, 'think(s) himself to be something, when he is not' (Gal. 6:3); or he is a slave, delivered into the hands of fear and, being of a childish mind, trembles and is afraid, where for those who fear God there is no fear or trembling."