It is odd English. The way I read it 'knocking' is used to mean something like 'hitting'. It is trying to break, or disrupt my spirit.

Nice explanation, Michael.

I don't think so, because then you would expect the response to be something like, "But it will not enter my spirit." When you knock at a door, you are trying to gain entry. A disease doesn't gain entry to a spirit, in any case. If by "knocking" is meant something like "striking," the response would also be something different than this. "Clutching at my spirit" would make more sense, perhaps. As it stands, these sentences don't go together or make much sense.