Romans 4

In trying to help the Jewish converts understand the doctrine of Christ and its relationship and justification through faith and works, Paul turns to the life of Abraham, one that all Jews revered, to teach his point. And to accomplish that he returns to the Jewish tradition (or works) of circumcision -

For if Abraham were justified by works, he hath whereof to glory; but not before God.
3 For what saith the scripture? Abraham believed God, and it was counted unto him for righteousness." (New Testament | Romans 4:2 - 3)

"...who also walk in the steps of that faith of our father Abraham, which he had being yet uncircumcised." (New Testament | Romans 4:12)

First and foremost, Abraham was a righteous man before circumcision because of his faith in the Lord. And even then, because of faith, he obeyed the commandments of the Lord including circumcision -

"...for we say that faith was reckoned to Abraham for righteousness." (New Testament | Romans 4:9)

Abraham was an example that faith and obedience go hand in hand. Circumcision then became a tradition or sign of remembrance of the covenants the Lord made with Abraham. But even this tradition or works had not the power of salvation. This power came only through faith on Jesus Christ and the atonement. True faith requires obedience. Performing the tradition or Mosaic law of circumcision was meaningless if it did not lead to obedience and faith.

It appears that even though Paul taught that obedience to the law of Moses or as he referred to as "works" was not necessary to righteous, he recognized that it may still lead to obedience and faith amongst the Jewish faithful. Of such though, Paul makes it perfectly clear -

"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.
14 For if they which are of the law be heirs, faith is made void, and the promise made of none effect..." (New Testament | Romans 4:13 - 14)

The great covenants and promises to Abraham, his seed, and faithful saints then is based conditioned upon righteousness and faith, not the Mosaic law with related works.