It could have telepathically plucked it from the back of Karras’s mind, and made sarcastic reference to it in that context to make him feel guilty about it/remind him that he was a sinner. It could definitely read minds (so well that you didn’t have to actually be thinking something in particular at a given time for it to be able to glean it from your head).
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It could ‘google’ human minds telepathically (so to speak), placing a lot of knowledge at it’s fingertips; in Karras’s presence it ‘knew’ classical Greek in addition to Latin, for example. The German it plucked from Karl’s mind, and so on. It could create the illusion of omniscience in this manner, but it wasn’t my impression while reading the book that it really possessed it.Â