James Perloff, author of ‘Missing Miracles, Missing Saints, mentioned this quote in a podcast.
Saint Peter the Egyptian (4th-5th century):
Some other time Peter was asked: ‘What is a servant of God?’ His answer was this: ‘As long as one practices any kind of passion he cannot be called servant of God, but he is the servant of that passion which conquers him. As long as he is conquered he cannot teach others who are conquered by the same passions, because it is shameful for him to teach before he has been freed from that passion about which he teaches.’ (source)
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Compare to Paul:
“Those who belong to Christ have crucified the flesh with its passions and lusts.” – Gal. 5:24