Our speaker is a creative guy. He arranges his poem in the shape of an altar, and tells God he's building an altar made out of his heart that is held together with tears. Nobody has carved or fixed up the "stones" he's using to build it (they are just the way God made them). He's putting all of his heart and soul into this altar in order to praise God, and in order to leave something behind after he's dead and gone that will still praise God. In the end, he wants God's sacrifice to be his, and for God to bless this altar he has built for him.