Romans 4:10

Under what circumstances was it credited? Was it after he was circumcised, or before? It was not after, but before!

(v10, NIV)

See, some Jews may point to the fact that Abraham was circumcised and that was credited to him as righteousness. However, if you look carefully, you'll notice that it was credited to him as righteousness before he was circumcised. Therefore, the argument doesn't hold anymore. Abraham had faith and it was not works that merited him any sort of justification.