Proverbs 17:3 Fire test the purity of silver and gold, but the Lord test the heart.
From what I understand, and if I am wrong please tell me! Gold melts at such high temperatures that the other thing in the gold melts first and that way you know that gold is not pure... but you can use heat to separate gold from the impurities to make the gold about 99% pure. You can also use heat to mold gold into new shapes. God knows our heart, God knows the impure things in our heart and God can test our heart to see what is there that doesn't belong. God can also remove the bad from our hearts by his grace... isn't that exciting if we expose our hearts to God, just like if we expose gold to heat, our hearts can get purer and purer... just like the longer you expose gold to heat the purer it gets, the longer we expose our hearts to God the purer our hearts get. Can we be 100% perfect, no... but we can do our best to be pure, we can expose our hearts to God and he can remold us into the people he wants us to be.