A.T. Robertson Commentary Romans 4:10

"How then was it reckoned? when he was in circumcision, or in uncircumcision? Not in circumcision, but in uncircumcision:" — Romans 4:10 (ASV)

When he was in circumcision (εν περιτομη οντ). Dative masculine singular of the present active participle of ειμ; "to him being in a state of circumcision or in a state of uncircumcision?" A pertinent point that the average Jew had not noticed.