'Promise' in the Bible

Again, the promise that he should inherit the world did not come to Abraham or his posterity conditioned by Law, but by faith-righteousness.

For if it is the righteous through Law who are heirs, then faith is useless and the promise counts for nothing.

so that the promise should be made sure to all Abraham's true descendants; not merely to those who are righteous through the Law, but to those who are righteous through a faith like that of Abraham. Thus in the sight of God in whom he believed, who gives life to the dead and makes reference to things that do not exist, as though they did, Abraham is the forefather of all of us. As it is written, "I have appointed you to be the forefather of many nations."

Nor did he in unbelief stagger at God's promise, but became mighty in faith, giving glory to God,

and being absolutely certain that whatever promise He is bound by He is able also to make good.

nor because they are Abraham's true children. But the promise was "Through Isaac shall your posterity be reckoned."

In other words, it is not the children by natural descent who count as God's children, but the children made such by the promise are regarded as Abraham's posterity.

For the words are the language of promise and run thus, "About this time next year I will come, and Sarah shall have a son."