This reminded me of a passage that has intrigued me for years in The Pilgrim’s Progress. It comes from section IV, where Christian is making his difficult way through the Valley of the Shadow of Death:
One thing I would not let slip; I took notice that now poor CHRISTIAN was so confounded, that he did not know his own voice. And thus I perceived it: just when he was come over against the mouth of the burning pit, one of the wicked ones got behind him, and stept up softly to him; and whisperingly suggested many grievous blasphemies to him--which he verily thought had proceeded from his own mind. This put CHRISTIAN more to it than anything that he met with before, even to think that he should now blaspheme him that he loved so much before! Yet could he have helped it, he would not have done it; but he had not the discretion neither to stop his ears, nor to know from whence those blasphemies came.
Sometimes I wonder if the awful thoughts come from a source other than that cesspool of remaining sin in our hearts. Bunyan captures the idea well here. I suppose we will never know exactly the source of these things, but in our self-examination, we do well to remember that there is an Enemy and that he does attack.