Produce fruit in keeping with repentance

Matthew 3:7-10 (or Luke 3:7-9) John the Baptist’s teachings

But when he saw many of the Pharisees and Sadducees coming to where he was baptizing, he said to them:

"You brood of vipers!
Who warned you to flee from the coming wrath?
Produce fruit in keeping with repentance.
And do not think you can say to yourselves,
'We have Abraham as our father.'
I tell you that out of these stones God can raise up children for Abraham.

Luke 1:17 Regarding John the Baptist

And he will go on before the Lord, in the spirit and power of Elijah, to turn the hearts of the fathers to their children and the disobedient to the wisdom of the righteous—to make ready a people prepared for the Lord."

Matthew 17:12-13 John the Baptist was Elijah

Jesus said to them, “But I tell you, Elijah has already come, and they did not recognize him, but have done to him everything they wished. In the same way the Son of Man is going to suffer at their hands." Then the disciples understood that he was talking to them about John the Baptist.

Romans 11:19-24

You will say then, "Branches were broken off so that I could be grafted in."
Granted. But they were broken off because of unbelief, and you stand by faith. Do not be arrogant, but be afraid. For if God did not spare the natural branches, he will not spare you either.

Consider therefore the kindness and sternness of God: sternness to those who fell, but kindness to you, provided that you continue in his kindness. Otherwise, you also will be cut off.
And if they do not persist in unbelief, they will be grafted in, for God is able to graft them in again.

After all, if you were cut out of an olive tree that is wild by nature, and contrary to nature were grafted into a cultivated olive tree, how much more readily will these, the natural branches, be grafted into their own olive tree!

Hebrews 3:12-19
See to it, brothers, that none of you has a sinful, unbelieving heart that turns away from the living God.

But encourage one another daily, as long as it is called Today, so that none of you may be hardened by sin's deceitfulness.

We have come to share in Christ if we hold firmly till the end the confidence we had at first.
As has just been said:

"Today, if you hear his voice,

do not harden your hearts

as you did in the rebellion."

Who were they who heard and rebelled?
Were they not all those Moses led out of Egypt?
And with whom was he angry for forty years?
Was it not with those who sinned, whose bodies fell in the desert?
And to whom did God swear that they would never enter his rest if not to those who disobeyed?
So we see that they were not able to enter, because of their unbelief.