How was it then reckoned? when he was in circumcision, or in uncircumcision? Not in circumcision, but in uncircumcision.
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What did Abraham believe? He believed that he would have a descendant, a son, in whom all the nations would be justified by faith while they were still uncircumcised, as Abraham then was. Commentary on Paul’s Epistles.
In effect, Abraham received circumcision a year only before the birth of Isaac; whereas he had received the promises and justification more than 25 years before, when the Almighty caused him to depart from Mesopotamia. (Calmet)
Therefore he was justified by faith and grace, which is common both to the circumcised and uncircumcised. (Menochius) ...