First the passage:
26 So in Christ Jesus you are all children of God through faith, 27 for all of you who were baptized into Christ have clothed yourselves with Christ. 28 There is neither Jew nor Gentile, Slave nor free, neither male nor female, for you are all one in Christ Jesus. (Fee's translation; 135)
Here are some select quotes from the always insightful Fee:
It is therefore especially difficult for most of us to imagine the effect of Paul's words in a culture where position and status preserved order through basically uncrossable boundaries. Paul asserts that when people come into the fellowship of Christ Jesus, significance is no longer to be found in being Jew or Greek, slave or free, male or female. The all-embracing nature of this affirmation, its counter-cultural significance, the fact that it equally disadvantages all by equally advantaging all--these stab at the very heart of a culture sustained by people's maintaining the right position and status. But in Christ Jesus, the One whose death and resurrection inaugurated the new creation, all things have become new; the new era has dawned.
...Second, "all of you are one." ...In Christ Jesus we no longer find our basic values in what differentiates people from one another, but in the unity that Christ alone can bring into our continuing diversity. It is one of the sad realities in the history of the church that so few of God's people have ever really caught on to what Paul is here asserting to be true: We are one people together, united in our common life in Christ (143-144; italics original)