I consider every gain I had made without Christ a loss, including circumcision. The two verses that precede today’s reading (Philippians 3:1-3) provide the basis and sets the stage for Paul’s contention in what follows. In those opening two verses, Paul warns the Philippians to beware of ‘self-mutilators,’ a term he uses to refer to those who advocate for circumcision. From what we read in Paul’s other letter (cf. Galatians 5:2), we would presume that this community had also been infiltrated by the false teaching that one has to be circumcised in order to be a bona fide member of the Christian family. In refuting such a teaching, Paul says that while circumcision was required of him in his former life, it is not required of him in his present life. While it was such a ‘good’ gain in his former life (it gave one status and privileges), that is not the case in his present life. As a matter of fact, he considers it a loss! 
Paul considers as a loss circumcision and any other thing done without Christ of for the cause of Christ. For all the privileges and ‘rights’ that were accorded one by them have been granted to all in Jesus Christ. In Christ, everybody has been made into a child of God, and there is nothing else required of men and women (other than faith in Christ) for them to access God. Could an individual still be circumcised without losing his status as a child of God? Of course. However, it should not be done as a requisite for salvation for otherwise it becomes a loss. In Christ, circumcision or un-circumcision means nothing (cf. Colossians 3:11; 1 Cor 7:19; Galatians 6:15).