The fact that Krista has Atheists on On Being lends validity to her claim to explore, through her podcast, "the big questions at the center of human life." If she only included Theists and the religious, it wouldn't be a valid exploration. Similarly, only including Atheists/Agnostics wouldn't be a valid exploration. Exploring all the ideas helps the listener decide for him/herself what to think and believe.
Bare in mind, this podcast was recorded at a Theology school. They knew what they were doing when they invited a non-religious person to speak there. Krauss sought to challenge the universally religious people present at the podcast, which is valid.
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