The scriptures tell us that Abraham believed God and it was accounted to him for righteousness. Specifically, Abraham believed God’s promise to make Him the father of many nations.
In Galatians 3:8 the apostle Paul says, “…the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, in thee shall all nations be blessed.”
God’s promise to Abraham, which Abraham believed and was justified, was the gospel, and it was a promise that would bless all nations!
In Romans 4, Paul refers to this promise as Abraham becoming the heir of the world.
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. ~ Romans 4;13
Notice that Paul makes a distinction between the Law and the righteousness of faith with regards to this promise. God’s promise wasn’t to Abraham alone. It was to Abraham and his seed, Christ, and those who are in Christ.
In Galatians 3:16 Paul says, Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
Jesus Christ is the seed through whom the promises God made to Abraham have been brought to fruition.
The Law was added some 430 years after God made His promise to Abraham, and it was added because of sin until the seed should come. Through His death for us, Jesus redeemed us from the curse of the Law and now the the promise of the Spirit (the promise made through the Spirit to Abraham) has come on the Gentiles.
God’s promise to Abraham was the covenant God made “in Christ.” In Christ, Abraham has become the father of many nations! ~ Galatians 3:17
For ye are all the children of God by faith in Christ Jesus. For as many of you as have been baptized into Christ have put on Christ. There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female: for ye are all one in Christ Jesus. And if ye be Christ’s, then are ye Abraham’s seed, and heirs according to the promise. ~ Galatians 3:26-29