But God said to Abraham, “Do not be distressed about the boy and your maidservant. Listen to everything that Sarah tells you, for through Isaac your offspring will be reckoned.
And the LORD said to Samuel, “Listen to the voice of the people in all that they say to you. For it is not you they have rejected, but they have rejected Me as their king. ... Now listen to them, but you must solemnly warn them and show them the manner of the king who will reign over them.”
I declare the end from the beginning, and ancient times from what is still to come. I say, ‘My purpose will stand, and all My good pleasure I will accomplish.’
But God replied, “Your wife Sarah will indeed bear you a son, and you are to name him Isaac. I will establish My covenant with him as an everlasting covenant for his descendants after him. ... But I will establish My covenant with Isaac, whom Sarah will bear to you at this time next year.”
Nor because they are Abraham’s descendants are they all his children. On the contrary, “Through Isaac your offspring will be reckoned.” So it is not the children of the flesh who are God’s children, but it is the children of the promise who are regarded as offspring.