You shall not hearken unto the words of that prophet, or that dreamer of dreams: for the LORD your God is testing you, to know whether you love the LORD your God with all your heart and with all your soul.
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Augustine of Hippo
He permits us to be tested not in order that he, for whom nothing lies hidden, might know but in order that he might make us know the extent of our progress in love for him. According to this same mode of speech our Lord also says that he does not know the day or the hour of the end of the world. What can there be that he does not know? He was concealing it from the disciples for their benefit, and he said that he did not know it because he was causing them not to know by concealing it. On Genesis, Against the Manichaeans.