Fire, in the spiritual sense, can mean either love or hatred depending on the context, just as natural fire can be either comforting in keeping you warm, or scary in burning down your house. Our language reflects this, too -- we use concepts like a smoldering hatred or a burning love. So fire signifies a love, either a good love of the neighbor and to the Lord, or, in a bad sense, selfish love of oneself that, if unchecked by conscience, leads to hatred of anyone that opposes it.

In Matthew 12:31-32, this signifies to speak against the literal sense of the Word, or the divine truth as it is on earth; while to speak against the Holy Spirit is to utter blasphemies against the Divine Truth as it is in the Lord, or in heaven. (Apocalypse Explained 778[3])