*spoiler* How did the occultist...

I assumed he died of an infection caused by his wound or just severe blood loss.

Responding to a seven year old post by someone with 34 posts in total, but...

In my interpretation of the movie, Solomon's death was a result of the ritual taking effect and the two of them getting their wishes. Indeed, when Solomon was stabbed he commented about how it was a sign indicating they were successful in completing the ritual. It is also important to note that after Solomon was stabbed, both he and Sophia were no longed participating in the ritual. That the ritual was successful indicates that the correct rites had been performed before Solomon was stabbed.

Solomon's wish was to be invisible. Though the idea of an invisible man walking around like Kevin Bacon in Hollow Man is the most obvious thing that comes to mind when referring to invisibility, death is also also a means to become invisible. In regards to reality, it's also the more logical.

So, to answer how did he die, Solomon's death was due to his wish being granted. The knife wound was the direct cause, but behind the cause of seemingly random event itself, where he stumbled onto the knife, was the completion of the ritual.