These verses pair groupings or concepts that separate people and keep them divided and sometimes at war with each other. Paul shows racial differences (Greek and Jew); religious differences (circumcised and uncircumcised); cultural differences (barbarian and Scythian); social differences (slave and free); and finally sexual difference (male and female).
These are in no way all the differences that divide humanity, but they give enough of a representation for God to make His point. He makes it clear that we cannot be united to Him and separated from our brother at the same time. To do something for or against a brother is to do it to Christ (Matthew 25:31-46). Because we, as brethren, are "in" Christ and He "in" us, we are one organism. John says if a man does not love his brother, he does not love God (I John 4:20)! This is serious business. We must be one with both.
The person who is truly converted is motivated, guided, inspired, led by, yielding to, and empowered by the radiant energy flowing from Christ, who lives and works in Him. It is almost as if Christ and His converted brethren are driven together because they share the same nature.
John W. Ritenbaugh
All in All