"The words "it was credited to him" were written not for him alone, but also for us, to whom God will credit righteousness-for us who believe in him who raised Jesus our Lord from the dead" (Romans 4:23-24)
Why was Paul so careful making sure he mentioned that the result of Abrahams faith extended to us? The Scriptures say that, "Abram believed the LORD, and he credited it to him as righteousness." ( Genesis 15:6 ). Paul is noting that the same credit of righteousness is extended to everyone who firmly relies in Jesus as Lord.
Abraham's faith was a conduit of the righteousness that God bestowed upon him, but his faith rested in the promise. The promise extends to us because the promise referred to the Messiah. Abraham saw a glimpse of this and knew that the Salvation of the world would come through his heir. Fortunate for us, God extends this hope of salvation to everyone who believes in Jesus. Romans 2:11 says, that God is no respecter of persons (KVJ). This is a great thing because it doesn't rely on who we are or what we do, it only relies on the person and the grace of Jesus.
There is a caveat however. To have righteousness credited unto your account you have to believe that God raised Jesus from the dead. To the natural man, this is utterly ridiculous and hard to understand. On the other hand, many people confess this, but their confession is merely words. The Amplified Bible explains what truly believing is... to believe you must firmly rely on, trust in and adhere to what you confess. We can take lesson from Abraham because he was fully persuaded in the promise and because of that any doubt that remain was overpowered by his faith. To have God's righteousness credited to us, we have to be fully persuaded that He raised Jesus from the dead, and we have to believe this by faith.
Today, I praise God that we have Abraham as a powerful example of faith. I praise God that the blessing he received extends to us. I am so thankful to have obtained this righteousness apart from the Law, which is by faith. The same faith working in all who believe in Jesus. What a marvelous thing!