Did the apostles believe in circumcision?

Yes, the apostles believed in circumcision as a sign of the covenant between God and Abraham. They did not consider it necessary for salvation, but they did not reject it either. For example, in Acts 15, the apostles and elders in Jerusalem debated whether or not Gentile converts to Christianity should be circumcised. They ultimately decided that circumcision was not necessary, but they did not condemn it either.