Abraham is the father of the physical nation God elected to work in and through, and he is also the father of those He calls to be part of His forming spiritual nation.

 Galatians 3:29

(29) And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.

  Romans 4:11-13

(11) And he received the sign of circumcision, a seal of the righteousness of the faith which he had yet being uncircumcised: that he might be the father of all them that believe, though they be not circumcised; that righteousness might be imputed unto them also: (12) And the father of circumcision to them who are not of the circumcision only, but who also walk in the steps of that faith of our father Abraham, which he had being yet uncircumcised. (13) For the promise, that he should be the heir of the world, was not to Abraham, or to his seed, through the law, but through the righteousness of faith.
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Abraham is the father of the physical nation God elected to work in and through, and he is also the father of those He calls to be part of His forming spiritual nation. Appreciating Abraham as the father of the physical nation is easy, but seeing him as father of the spiritual nation is not so simple, as we are more apt to think that, because God begets us, He is our spiritual Father. We must therefore see Abraham's spiritual fatherhood in a different light.

The Jews of Jesus' day did not grasp Abraham's spiritual fatherhood correctly. Jesus gives the answer to this perplexing title in John 8, where a great deal of the dialogue involves ancestry.

"And yet if I do judge, My judgment is true; for I am not alone, but I am with the Father who sent Me. . . . I am One who bears witness of Myself, and the Father who sent Me bears witness of Me." They said to Him, 'Where is Your Father?" Jesus answered, "You know neither Me nor My Father. If you had known Me, you would have known My Father also." . . . Then Jesus said to them, "When you lift up the Son of Man, then you will know that I am He, and that I do nothing of Myself; but as My Father taught Me, I speak these things. And He who sent Me is with Me. The Father has not left Me alone, for I always do those things that please Him." (verses 16, 18-19, 28-29)

Abraham's spiritual fatherhood soon becomes the focus of Jesus' instruction:

They answered Him, "We are Abraham's descendants, and have never been in bondage to anyone. How can you say, 'You will be made free'?" . . . "I know that you are Abraham's descendants, but you seek to kill Me, because My word has no place in you. I speak what I have seen with My Father, and you do what you have seen with your father." They answered and said to Him, "Abraham is our father." Jesus said to them, "If you were Abraham's children, you would do the works of Abraham. But now you seek to kill Me, a Man who has told you thetruth which I heard from God. Abraham did not do this." (John 8:33, 37-40)

Members of the same family bear a likeness to one another. It is this principle that designates who is a spiritual child of Abraham. It is not a matter of physical resemblance but a similarity of moral and spiritual attitude and behavior. Christ designates that resemblance to be believing God as Abraham did, as well as doing the works that he did. In the larger picture, a spiritual descendant of Abraham will grow and overcome, gradually changing into the spiritual image of Jesus Christ.

The factor that set Abraham apart above all others was that faith drove, motivated, inspired, and guided—sometimes dramatically—what he did with his life. Thus, Abraham is not only the physical progenitor of Israelites but also the spiritual, moral pattern that his descendants are to conform to.

— John W. Ritenbaugh