For the promise to Abraham or to his descendants that he would be heir of the world was not through [observing the requirements of] the Law, but through the righteousness of faith.

If those who are [followers] of the Law are [the true] heirs [of Abraham], then faith [leading to salvation] is of no effect and void, and the promise [of God] is nullified.

Therefore, [inheriting] the promise depends entirely on faith [that is, confident trust in the unseen God], in order that it may be given as an act of grace [His unmerited favor and mercy], so that the promise will be [legally] guaranteed to all the descendants [of Abraham]—not only for those [Jewish believers] who keep the Law, but also for those [Gentile believers] who share the faith of Abraham, who is the [spiritual] father of us all—

But he did not doubt or waver in unbelief concerning the promise of God, but he grew strong and empowered by faith, giving glory to God,

and they are not all the children of Abraham because they are his descendants [by blood], but [the promise was]: “Your descendants will be named through Isaac” [though Abraham had other sons].

That is, it is not the children of the body [Abraham’s natural descendants] who are God’s children, but it is the children of the promise who are counted as [Abraham’s true] descendants.

For this is what the promise said: “About this time [next year] I will come, and Sarah shall have a son.”