TSK · Galatians 4:28

You are the children of the prophets, and of the covenant which God made with our fathers, saying to Abraham, 'In your seed will all the families of the earth be blessed.'

That is, it is not the children of the flesh who are children of God, but the children of the promise are counted as a seed.

However, the son by the handmaid was born according to the flesh, but the son by the free woman was born through promise.