The virgin soul that desires to be united to God must keep itself pure not only from overt sins like unchastity, murder, theft, gluttony, backbiting, falsity, avarice, greed and so on; but to an even greater degree it must keep itself pure from sins that are hidden, such as desire, self-esteem, love of popularity, hypocrisy, love of power, wiliness, malice, hatred, unbelief, envy, self-love, affectation and other things of this kind. According to Scripture, these concealed sins of the soul are just as pernicious as the overt sins.
–Saint Macarius the Great (295-392)