"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" (vv. 6–7a).
If we were to ask the average, biblically literate person the identity of those to whom the prophets were sent, in most cases we would hear this answer: "ancient Israel." Of course, that is correct, but it needs further clarification. When we say the old covenant prophets brought God's message to Israel, we are actually talking about two different, sometimes-but-not-always overlapping groups of people.
Scripture uses the term Israel in two major ways. First, it can mean the nation made up of every physical descendant of Jacob. "The people of Israel" in Exodus 12:33–42 refers to all of Jacob's progeny then living in Egypt—regardless of whether they trusted Yahweh. This use of Israel applies to what theologians call the "visible covenant community." The qualifier visible reflects our ability to see, so the visible covenant community under the old covenant consisted of everyone whom human beings could identify as a citizen of Israel by outward markers such as family ties and circumcision.
Yet the Bible also uses the word Israel to refer to those who were Israelites inwardly— to all those who had saving faith in Yahweh during the old covenant era. Paul the Apostle uses the word Israel in this way in today's passage. This use of Israel corresponds to what theologians have historically termed the "invisible covenant community." Again, the qualifier invisible applies to our sight. We cannot see the state of another person's heart, so we do not know in every instance that the one who professes saving faith actually possesses it. What God sees, of course, is another matter. He always knows when a person who professes faith as a member of the visible covenant people also possesses faith as a member of the invisible covenant people.
When speaking to Israel, the Old Testament prophets delivered God's Word to the visible covenant community. Thus, even when they declared that Israel had broken its covenant with God, they knew a remnant of Israel still trusted Him. This distinction also reveals that the prophetic promises that so often seem to refer unconditionally to all the physical descendants of Jacob cannot be read that way. The prophets spoke to the visible covenant community, so they had to preach in universal terms. However, their words always assumed that only true Israelites would inherit God's promises. The ultimate covenant blessings were for only those who had saving faith in the Lord.
Coram Deo
God's Word is delivered to all of the members of the visible covenant community. But the members of the invisible covenant community show themselves to be His elect people by taking the warnings about staying in faith seriously, repenting, and persevering in trusting God until the ends of their lives. If we do not take the words of the Lord seriously and soberly apply them to our lives, we reveal that we may be members only of the visible covenant community.