God tells Abram to head toward a different land, which is linked with his people becoming a great nation. We usually interpret this as meaning a vast number of physical descendants, and God has certainly fulfilled that, considering the teeming populations of his offspring. However, the real meaning of being Abraham's children has to do with those who have the faith of Abraham (Galatians 3:7).
The Jews boasted that Abraham was their father, yet they were concerned only with physical lineage. Jesus told the priests and Pharisees that the kingdom would be taken from them and “given to a nation bearing the fruits of it” (Matthew 21:43). That nation is defined, not by a physical bloodline, but by a certain faith and a different spirit. Peter calls those with the faith of Abraham “a chosen generation, a royal priesthood, a holy nation, His own special people” (I Peter 2:9).
Genesis 12:3 says that in Abraham “all the families of the earth shall be blessed.” Paul explains this promise in Galatians 3:8: “And the Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel to Abraham beforehand, saying, 'In you all the nations shall be blessed.'” From the Genesis 12:3 promise, Paul derives the idea that justification by faith would become available. In addition to foretelling a spiritual nation, God's promise of the land also suggests many being brought into alignment with God's standard of righteousness based on belief in Him.
— David C. Grabbe