'Momentaneous salvation from immediate mercy' is represented by the fiery flying serpent in the church in Isaiah 14:29. This serpent symbolizes people who adhere to the doctrine of salvation by faith alone.

It is worth looking at Divine Providence 340 to gain some more perspective on this passage: "This kind of evil is on the wing in the church whenever it believes in instant salvation by direct mercy, because this (a) leads to the destruction of religion, [because it leaves out the need for real, sustained repentance] (b) makes people feel safe, and (c) blames God for damnation.

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.