Abraham represents Faith. He Believed God and it was counted for Righteousness .
Sarah represents Grace. God's unmerited favor was given to her so that she became the mother of many people including our Lord .
Abraham and Sarah were half-siblings. They proceeded from a single parent.
Abraham provided the Seed. Sarah provided the Abundance .
Faith has to be in Grace in order for the increase to occur.
Grace and Faith together is Graith .