Your friend saw this resource on bethinking.org and thought you should see it:
Relativism: the loss of 'truth'
Two sample chapters from
Relativism: Feet Firmly Planted in Mid-Air by Greg Koukl and Francis J. Beckwith.
See the full resource online by visiting bethinking.org:
https://www.bethinking.org/truth/the-death-of-truth
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bethinking.org is a collection of resources which aims to make sense of the big questions of life about God, Jesus and Christianity.