While much of the verse is devoted to a description of the devil, or Satan, the reason for the description is that such would be our fate were there no resurrection. Without the possibility of the resurrection of the body "our spirits must have become like unto him, and we become devils."
It would be easier to understand this condition if Jacob were speaking of the atonement for sin. Certainly if we were unable to repent, our sins would eventually bow us down to the level of Satan. However, here Jacob is relating this only to the separation of body and spirit. He will pick up on the theme of the atonement from sin in the next few verses.