separate the sheep from the goats
Distinguish between good and bad individuals, or superior and inferior ones. For example, In a civil war where both sides commit atrocities, you can't separate the sheep from the goats. This term refers to Jesus's prophecy in the New Testament (Matthew 25:32) that the sheep (that is, the compassionate) will sit on God's right hand (and find salvation), and the goats (the hard-hearted) will sit on the left (and be sent to damnation).
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