Nor are they all children because they are the seed of Abraham; but, “In Isaac your seed shall be called.” That is, those who are the children of the flesh, these are not the children of God; but the children of the promise are counted as the seed.
The apostle Paul uses the fact that God had not always passed the blessings of Abraham on to the firstborn son to make a point. God Himself determines who will receive His blessings and when. In this letter to the Romans, Paul is proving that God still loves the Jews and will eventually save Israel, but that He was now making salvation available to the gentiles as well.
God loves all of mankind, and as He said to Moses, “I will have mercy on whomever I will have mercy, and I will have compassion on whomever I will have compassion” (Romans 9:15).
For more about this passage, see “All Israel Will Be Saved!”