His Offspring - Romans 9:6-8

Romans 9:6-8 ESV 

But it is not as though the word of God has failed. For not all who are descended from Israel belong to Israel, and not all are children of Abraham because they are his offspring, but “Through Isaac shall your offspring be named.” 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 Israel's descendants belong to Israel, but some do. Not all of those who call on the name of the Lord are in the body of Christ, but some are. 

Some in Israel put their faith in God and followed His commandments. Some followers of Christ place their faith in Him and follow His commandments. 

These are the children of promise, of the promise of God, and are His offspring.

Matthew 7:21-23

“Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but the one who does the will of my Father who is in heaven. On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’ And then will I declare to them, ‘I never knew you; depart from me, you workers of lawlessness.’