If you rewatch it Ragnar fell down and after he got back up he threw his axe at someone looking like Jarl Borg from behind :). Vikings likes to play around with the supernatural a lot and sets out to leave the viewer in a constant state of confusion on if something was supernatural or not. Pretty much every time we see something that seems supernatural the person already has a bias and is in a state of pain/shock/trauma. In this example Athelstan has just been fucking cruzified and is now emotionally torn between his two beliefs, both of which mean very much to him. So yeah maybe he just saw the devil under his bed or maybe it was a snake or something and Athelstan is just a little out of it atm.