According to John, the true children of Abraham are those who are repentant and are his children by faith.
John the Baptist does not say that God can merely bypass Abraham’s descendants, but rather that he can raise up other children of Abraham out of these stones. Descent from Abraham is therefore no guarantee, but it does impose an obligation. What matters is whether one is truly a child of Abraham by faith and whether one is repentant (see Romans 9:6). If you are not born of God, you have no right to enter the kingdom of heaven. This message must have been shocking to the Pharisees and the Sadducees, even though it logically flows from the Old Testament (Hosea 1:10–11, Isaiah 55:1–56:8).1
It is God who generates the children of Abraham, making them children of the promise (see Romans 4:13–25 and Galatians 3:6–9). This is explained by John in a very distinct manner. God made Adam out of the dust of the earth, and he called Abraham when he was still only an individual with no descendants (Isaiah 51:1–2). John wants the Pharisees and Sadducees to understand that Israel will be judged lest it repents, even if they are the physical descendants of Abraham (Malachi 3:2–5). It is not them, but rather whoever God predestined, who belongs to the true descendants of Abraham. This spiritual progeny is not generated by itself, but by divine power.
9 And do not presume to say to yourselves, ‘We have Abraham as our father,’ for I tell you, God is able from these stones to raise up children for Abraham.