Romans 4:11

And he received circumcision as a sign, a seal of the righteousness that he had by faith while he was still uncircumcised. So then, he is the father of all who believe but have not been circumcised, in order that righteousness might be credited to them.

(v11, NIV)

So, if Abraham was justified by faith, what was the purpose of his circumcision? That's a legitimate question if Paul is making the point that Abraham was not justified by works. Well, apparently circumcision was received as a sign. Sure, Abraham had received righteousness by faith, but circumcision was a sign of that in a sense. It's described as a seal, and I am reminded of how the Holy Spirit is a seal or guarantee for us as described in Ephesians 1. It's not what justifies but it was like proof for Abraham. This doesn't mean it applies the same way for us. Just because you're physically circumcised doesn't mean you have been declared righteous according to faith.