Abraham is the father of Isaac and the grandfather of Jacob, who became Israel, who generated twelve sons who became the twelve tribes of Israel. And yet before he had borne a single child, God made a promise to him that his seed would be multitudinous and great. "Now Abram believed on Elohim, and He reckoned it to him for righteousness" (Genesis 15:6).

For our apostle Paul, this became important Scriptural recourse for his gospel of race-wide justification by faith. It set forth faith and faith alone as a precedent for blessing.

Why does Paul push Abraham in Galatians? Because the Judaizers  are pushing Moses. Paul needs to trump them. Paul's "ace in the hole" is this: God's promise to Abraham, based on faith, came 430 years BEFORE the law of Moses. This promise was ratified by God, and therefore the law of Moses does not invalidate the promise. It's not that the nations now become part of Abraham's seed destined to become a priesthood, but that there is a precedent for blessing via faith apart from works.