Ouketi hos does not mean "not only as" or "not merely as." Here it means "no longer as." The ouketi...alla ("no longer...but") contrast emphasizes that the former condition is to stop and that the latter condition exceeds and supersedes it. To suggest that receiving Onesimus "no longer as a slave" has no social implications and that Paul is merely saying what we hear in 1 Corinthians 7 (the Christian slave is actually the Lord's freedman) is to underestimate totally the force and content of Paul's rhetoric here. There was already a sentiment among the Stoics that all persons were created equal by God....Paul certainly believes that all persons in Christ are new creatures and of equal sacred worth. This clearly has implications for the way he treats Jews and Gentiles and men and women, and there is no reason to doubt it would have social implications for his views about relationships between Christian slaves and masters. Paul is perhaps here offering a statement of fact: when God sets people free, they are free indeed, no matter how people view them. Paul seeks, then, to persuade Philemon to view the matter from God's point of view. "The reality of the world as seen from within the world is replaced by the reality seen from within the church. For Paul there is only one reality."
(the quote at the end is from N. R. Petersen) (pages 79-80)
Ben Witherington III on Philemon 15, 16 from his book, The Letters to Philemon, the Colossians, and the Ephesians: A Socio-Rhetorical Commentary on the Captivity Epistles