The Oil of His Grace

From Pilgrim's Progress:

Then I saw in my dream, that the Interpreter took Christian by the hand and led him into a place where was a fire burning against a wall, and one standing by it, always casting much water upon it, to quench it; yet did the fire burn higher and hotter.

Then said CHRISTIAN, What means this?

The INTERPRETER answered, This fire is the Work of Grace that is wrought in the heart. He that casts water upon it, to extinguish and put it out, is the Devil; but in that thou seest the fire burn higher and hotter, thou shalt also see the reason of that. So he had him about to the back side of the wall, where he saw a man with a Vessel of Oil in his hand, of which he did continually cast (but secretly) into the fire.

Then said CHRISTIAN, What means this?

The INTEPRETER answered, This is Christ, who continually with the Oil of his Grace maintains the work already begun in the heart: By the means of which, notwithstanding what the Devil can do, the souls of his people prove gracious still. And in that thou sawest, that the Man stood behind the wall to maintain the Fire; this is to teach thee, That it is hard for the Tempted to see how this Work of Grace is maintained in the soul.