"Fire tests the purity of silver and gold, but the Lord tests the heart.” Proverbs 17:3
The way that you purify metals is to heat them and burn off the impurities. Figuratively, God talks about purifying us through a Refiner’s Fire where the life impurities are burned off of us. The refiner checks the purity of the metal by looking into the molten metal to see how clear a reflection of themselves they can see. When God looks at you and into your life, how clear a reflection of Himself does He see?