“You are of your father the devil, and your will is to do your father’s desires. He was a murderer from the beginning, and does not stand in the truth, because there is no truth in him. When he lies, he speaks out of his own character, for he is a liar and the father of lies. But because I tell the truth, you do not believe me” (vv. 44–45). – John 8:41–47
But as we see in the New Testament, many of them believed that mere physical descent was sufficient to make them children of the patriarch (Luke 3:7–9). Scripture, however, tells us that the true children of Abraham and the inheritors of his blessing are those who do what Abraham did, namely, rest on the promises of God alone. To be a true child of Abraham is to share his spiritual condition, to trust in Jesus as Messiah. Physical descent from Abraham without faith in Christ doesn’t make one a child of Abraham, while sharing his faith without being descended from him makes one the patriarch’s true child (Gal. 3:1–14).