He received the sign of circumcision as a seal of the righteousness that he had by faith while he was still uncircumcised. The purpose was to make him the father of all who believe without being circumcised, so that righteousness would be counted to them as well, (ESV)
Father Abraham had many sons. In the racial sense he is the father of all the Jews. Spiritually, he was the father of all the uncircumcised Gentiles who had placed their faith in God. The reality is that it was faith that unites all of Abraham's family. It was never about circumcision. It was about a change of the heart that followed after God.
The Jews loved to look back to Abraham as their father. They thought that they were good sons of Abraham if they had been circumcised. Yet, their hatred for the Gentiles, and their hatred for their own brothers demonstrated that the circumcision they received was only a physical act. It had nothing to do with their heart.
Perhaps you know some people who have engaged in physical activity that identifies them to some as belonging to Christ. They may have been baptized. They could have perfect church attendance. They might even claim to be a part of the family of God. The bigger question is, "Has their heart been changed?"
Many of Abraham's sons by race were not part of the family of faith. They had physical marks of circumcision, but no change of heart. No faith. There are also many uncircumcised who claim to be part of the family of faith who demonstrate by their actions that they do not know God. They are not part of the family.
Do you want to be a part of the family of faith? You must believe in God and accept his sacrifice through Christ on your behalf. Circumcised or not. Those who belong to the family of faith have a changed heart.