Albert Barnes Commentary Matthew 3:9

"and think not to say within yourselves, We have Abraham to our father: for I say unto you, that God is able of these stones to raise up children unto Abraham." — Matthew 3:9 (ASV)

And think not to say, etc. They regarded it as sufficient righteousness that they were descended from so holy a man as Abraham. They deemed it such an honor that it would go far to justify all his descendants (John 8:33–37, 53).

John assured them that this was a matter of small consequence in the sight of God. From the very stones of the Jordan he could raise up children to Abraham.

The meaning seems to be this: God, from these stones, could more easily raise up those who should be worthy children of Abraham, or be like him, than make you (who are proud and hypocritical) subjects of the Messiah's kingdom simply because you are descendants of Abraham.

Or, mere nativity, or the privileges of birth, avail nothing where there is no righteousness of life.

Some have supposed, however, that by these stones he meant the Roman soldiers, or the heathen, who might also have attended his ministry; and that God could raise up children to Abraham from them.