Grace had never been to the Tree of Souls and witnessed Eywa jack into the Na'vis' queues, she'd (probably) never seen any display of intelligence like the woodsprites performed for Neytiri regarding Jake, and she'd definitely never wrapped her queue around the Tree of Voices and heard the voices of ghosts. So that's why her belief was tepid: she hadn't had the same caliber of exposure as the Na'vi did. So, for her, there obviously was a strong element of "faith" involved, much more than for the Na'vi. When Eywa extracted her consciousness, it must've been really vindicating for her as a scientist....maybe it felt for her like when somebody with a strong faith in God here dies and goes to Heaven! But, again, the Na'vi have a much stronger interaction with Eywa, so there's no element of faith regarding Eywa's existence for them like there is for us about God's existence.
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