It is in the context of his discussion of Christian conduct that Paul gives a succinct summary of the unity of all believers in Christ: 

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          Here there is no Greek or Jew, circumcised or uncircumcised, barbarian, Scythian, slave or free, but Christ is all, and is in all (Col 3:11).

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          This concept is critically important to Paul, and to an understanding of the racial teachings of Christianity.  He repeats a variation of this verse in two other epistles:  1 Cor 12:13, Gal 3:28.  In Colossians, he casts the net wider, to include barbarians and uncircumcised, but he leaves out male/female.  The barbarians (Gk: "barbaros") referred to uncivilized non-Greek-speaking Gentiles.   Because Paul varied the categories in these verses, it means they are not meant to be hard and fast.  The point is in the finale: Christ is all and in all.

Thus the new life imparted by the Spirit to the believer puts to death the old "carnal" self, the "earthly nature," and supersedes one's ethnic (and sexual) identity.  This is the other aspect of the "mystery of God" which is hidden from the world.  Point 1 of this mystery was that the New Covenant in the blood of Christ extends to the Gentiles.  Point 2 was that all social distinctions and ranks have been relativized in the fellowship of the Spirit.  

Unfortunately, this second principle seems to have been hidden from many believers throughout Christian history.  Religious racism contradicts the core message of the Gospel.  Its presence in a congregation or an individual is evidence that they have not been truly regenerated by the Spirit of God.  This is made plain by John:

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          If anyone says, "I love God," yet hates his brother, he is a liar.  For anyone who does not love his brother, whom he has seen, cannot love God, whom he has not seen (1 Jn 4:20).