Eph 2:11 Therefore remember that you, once Gentiles in the flesh—who are called Uncircumcision by what is called the Circumcision made in the flesh by hands—
The contrast between the past and the present is outlined by the apostle. Remember that he is writing to believers bringing them through the journey of what God has done for and in them through Christ by the power of the Holy Spirit.
He starts with the division that existed between the Jews (circumcised) and the Gentiles (uncircumcised). The Jews despised the uncircumcised especially the Samaritans. A little history, relations, in the ancient world, between Jews and Samaritans were undoubtedly strained. The Samaritans were half-Jews and half-Gentiles. When Assyria captured the northern kingdom of Israel in 721 B.C. some of the Hebrews were taken into captivity while others were left behind. The ones left behind intermarried with the Assyrians. Thus these people were neither fully Hebrews nor fully Gentiles. Even in the time of Jesus, the Jews and the Samaritans still had no dealings with one another for this reason.
What makes this divide painful was that they were of the same father, Jacob. This prejudice, as in our time, had many forms such as unreasonable feelings, opinions, or attitudes, especially of a hostile nature, regarding an ethnic, racial, social, or religious group. It was important for Paul to bring such remembrance to strengthen their faith, increase gratitude, and foster ground for greater unity. His message as he starts is that we are now one in Christ. The wall of separation has been done away with by God in Christ.