Abraham didn’t just believe in God in general; he believed a specific promise God had made, and he adjusted his life around it.
And because Abraham “was fully convinced that what God had promised, he was also able to do it…it was credited to him for righteousness (Romans 4:21-22).”
Tim Keller says, “Saving faith is not believing that God is there. Further, it is not believing in a God who saves. It is believing God when He promises a way of salvation by grace.”
Faith’s object is the promise of God. Faith is believing that God will do what God said He’d do and adjusting your life around that promise.
Paul goes on to make the bridge to us in 4:23-25, “And when God counted him as righteous, it wasn’t just for Abraham’s benefit. It was recorded 24 for our benefit, too, assuring us that God will also count us as righteous if we believe in him, the one who raised Jesus our Lord from the dead. 25 He was handed over to die because of our sins, and he was raised to life to make us right with God.”
Just like Abraham believed God WOULD send a son that would bring salvation into the world as he was promised, we believe that Jesus IS that Son.