In the book of Philemon, Paul had warned slave owners that they had a responsibility towards their slaves and showed slaves as responsible moral beings who were to fear God. Paul did not condemn slavery, but he presented Onesimus as a Christian brother instead of a slave. When an owner can refer to a slave as a brother, the slave has reached a position in which the legal title of slave is meaningless.
The early church did not attack slavery directly but it laid the foundation for a new relationship between owner and slave. Paul attempted to unite both Philemon and Onesimus with Christian love so that emancipation would become necessary. Only after exposure to the light of the gospel could the institution of slavery die.