Counted As Offspring

Romans 9:6-8 (ESV Strong's)

6 But it is not as though the word of God has failed. For not all who are descended from Israel belong to Israel, 7 and not all are children of Abraham because they are his offspring, but “Through Isaac shall your offspring be named.” 8 This means that it is not the children of the flesh who are the children of God, but the children of the promise are counted as offspring.

Not all of the children of Abraham are counted as his offspring who are the children of the promise. Adam was the first son of God, and his descendants will inherit the promised land. Jesus is the second Son of God, and His offspring will inherit the kingdom of heaven. 

Isaac was the son of promise. Jesus was the promised one who is the Son, the seed of Abraham. We are the harvest from that seed.