“For what saith the scripture? Abraham believed God, and it was credited to him as righteousness.” – Romans 4:3
How did Abraham believe in Christ?
Galatians 3:8-9 says: “And 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.
So then they which be of faith are blessed with faithful Abraham”.
Abraham believed that through his seed, the whole nations of the earth would be blessed. That seed was Christ. In Christ, all the nations of the earth would be blessed – meaning, they would be made righteous.
Abraham was declared righteous by believing this, as Romans 4:3 says, “…Abraham believed God, and it was credited to him as righteousness”. Now also, anyone who believes or has faith in Christ as Abraham did is a child of Abraham – and is declared righteous just like he was.
So, we see that Abraham did not just believe that he would have just a child (Isaac), he believed in the promise of the yet-to-come Messiah! Like I always say, the saints in the Old Testament were justified by believing in Christ and what he would do. We in the New Testament are saved when we believe in what Christ has done.