Christ was qualified as a descendant. He took actions that demonstrated his trust in God practically daily. He trusted in God and God's purpose for Himself. He said "not my will but yours be done." This is the truest essence of "the gospel." He believed in God's purpose for Himself and was obedient even to death to fulfill it. "The gospel" is trusting in Christ and his purpose. Jesus was already a son of God and a son of Abraham... He didn't need to qualify as either because he was born as both. Someone who is not the son of God certainly needs to qualify as a son of God, and any gentile needs to be adopted as a son of Abraham - for one must have a heavenly witness and an earthly witness.
Deu 30:19 NASB95 - 19 "I call heaven and earth to witness against you today, that I have set before you life and death, the blessing and the curse. So choose life in order that you may live, you and your descendants,
Rom 8:15-17 NASB95 - 15 For you have not received a spirit of slavery leading to fear again, but you have received a spirit of adoption as sons by which we cry out, "Abba! Father!" 16 The Spirit Himself testifies with our spirit that we are children of God, 17 and if children, heirs also, heirs of God and fellow heirs with Christ, if indeed we suffer with [Him] so that we may also be glorified with [Him.]
The human choice God sets before us between life and death and commands us to choose is predicated on a synergistic heavenly witness and an earthly one. This is the condition for adoption. Through faith in the gospel of Jesus Christ we become sons of God and of Abraham. Once we are qualified as sons of Abraham, we receive the righteousness of Christ as an inheritance because of God's promise to Abraham that he and his descendants would be credited with righteousness.
Adoption is how we become known by God.
Mat 7:23 NASB95 - 23 "And then I will declare to them, 'I never knew you; DEPART FROM ME, YOU WHO PRACTICE LAWLESSNESS.'
Gal 4:5-9 NASB95 - 5 so that He might redeem those who were under the Law, that we might receive the adoption as sons. 6 Because you are sons, God has sent forth the Spirit of His Son into our hearts, crying, "Abba! Father!" 7 Therefore you are no longer a slave, but a son; and if a son, then an heir through God. 8 However at that time, when you did not know God, you were slaves to those which by nature are no gods. 9 But now that you have come to know God, or rather to be known by God, how is it that you turn back again to the weak and worthless elemental things, to which you desire to be enslaved all over again?
We must be qualified as a descendant of Abraham for salvation because the promises were made to Abraham. To inherit through those promises we must be qualified as Abraham's descendant, and not only sons of God.
Luk 19:9 NASB95 - 9 And Jesus said to him, "Today salvation has come to this house, because he, too, is a son of Abraham.
Heb 2:16 NASB95 - 16 For assuredly He does not give help to angels, but He gives help to the descendant of Abraham.
Furthermore, once we accept this Biblical truth we find many of the petty, lingering arguments within the body of Christ fall away like chaff in the wind. This resolves the Calvin vs Arminius debate, this resolves dispensationalism vs covenant theology, this resolves works vs faith, this resolves the enmity between Jew and Christian, this resolves Original Sin vs Original death, this resolves dual covenant theology and replacement theology, this even indirectly resolves ultimate reconciliation vs eternal damnation debates.
So seeing how much good it does, and that it is Biblically sound exegetical doctrine, I would ask you again what are you so afraid of in believing that those with faith in the gospel are qualified as Abraham's descendants and heirs according to the promises given to him? Do you have a sacred cow in this list of conflict resolutions?