Look back at Abraham your father, and Sarah who gave birth to you. When I called him, he was only one man, but then I blessed him and he had many descendants.
‘Father Abraham,’ he called out, ‘Have mercy on me and send Lazarus to dip the tip of his finger in water and cool my tongue, because I'm burning in agony.’
“Brothers and fathers, listen to me!” Stephen replied. “God in his glory appeared to our father Abraham when he was living in Mesopotamia, before he moved to Haran.
He is also the father of circumcised Jews not merely because they're circumcised, but because they follow the example of the trust in God our father Abraham had before he was circumcised.
So the promise is based on trusting God. It is provided as a free gift, guaranteed to all the children of Abraham—not merely to those who follow the law, but also to those who trust like Abraham, the father of us all.
Abraham trusted God when he was tested and offered Isaac to God. Abraham, who had accepted God's promises, was still ready to offer to God his only son,
Someone may argue, “You have your trust in God; I have my good deeds.” Well, show me your trust in God without good deeds, and I will show you my trust in God by my good deeds!