Rom 4:3 For what saith the scripture? Abraham believed God, and it was counted unto him for righteousness.

There are those, who teach righteousness by faith alone, twisting Paul's letters.
Speaking here
Rom 4:1 What shall we say then that Abraham our father, as pertaining to the flesh, hath found?
Rom 4:3 For what saith the scripture? Abraham believed God, and it was counted unto him for righteousness.

but when we read further, taking this verse in context, we find:
Rom 4:10 How was it then reckoned? when he was in circumcision, or in uncircumcision? Not in circumcision, but in uncircumcision.
Rom 4:11 And he received the sign of circumcision, a seal of the righteousness of the faith which he had yet being uncircumcised: that he might be the father of all them that believe, though they be not circumcised; that righteousness might be imputed unto them also:

Abraham received the promise, being uncircumcised,
Rom 4:13 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.

So, when we believe, we too are justified by faith, and receive the circumcision, but of the heart. We too are justified before God.
This was a covenant, and God told Abraham, that all male children who were not circumcised would be cut off, for he hath broken HIS covenant.
So this promise came with a price. And even though Abraham was justified by faith, he still had to preform the requirement of this promise.

BUT we can't stop there though, for even though Abraham was found to be justified for believing God about being heir to many nations, PRIOR to being circumcised, we move on years following:

And even though Abraham was made righteous for his faith, prior to the circumcision, God still tempted him.
Gen 22:1 And it came to pass after these things, that God did tempt Abraham, and said unto him, Abraham: and he said, Behold, here I am.
Gen 22:2 And he said, Take now thy son, thine only son Isaac, whom thou lovest, and get thee into the land of Moriah; and offer him there for a burnt offering upon one of the mountains which I will tell thee of.

Now we know Abraham was made righteous by his faith, but he still had to perform what was required, otherwise, that faith would have been dead.

For HE told Abraham

Gen 17:1 And when Abram was ninety years old and nine, the LORD appeared to Abram, and said unto him, I am the Almighty God; walk before me, and be thou perfect.

And Abraham KNEW God's law and commandments:

Gen 26:4 And I will make thy seed to multiply as the stars of heaven, and will give unto thy seed all these countries; and in thy seed shall all the nations of the earth be blessed;
Gen 26:5 Because that Abraham obeyed my voice, and kept my charge, my commandments, my statutes, and my laws.

What was the promise God gave Abraham?

Gen 15:4 And, behold, the word of the LORD came unto him, saying, This shall not be thine heir; but he that shall come forth out of thine own bowels shall be thine heir.
Gen 15:5 And he brought him forth abroad, and said, Look now toward heaven, and tell the stars, if thou be able to number them: and he said unto him, So shall thy seed be.
Gen 15:6 And he believed in the LORD; and he counted it to him for righteousness.

Then, as posted above:
Gen 26:4 And I will make thy seed to multiply as the stars of heaven, and will give unto thy seed all these countries; and in thy seed shall all the nations of the earth be blessed;
Gen 26:5 Because that Abraham obeyed my voice, and kept my charge, my commandments, my statutes, and my laws.

Why was God going to make his seed to multiply?
Because that Abraham obeyed my voice, and kept my charge, my commandments, my statutes, and my laws.