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"During this age in which the sinfulness of the human heart remains even among God's people, and in which the temptation to self-exaltation and self-sufficiency is relentless, God has appointed that his servants tremble with a profound sense of insufficiency so that we will never forget that it is God's power and not man's wisdom which creates and sustains saving faith. "
 
http://www.soundofgrace.com/piper.htm