Philemon 1:16

no longer as a servant, but more than a servant, a brother beloved, specially to me, but how much rather to thee, both in the flesh and in the Lord.

The new relationship did not mean that Onesimus would be no longer a slave in the legal sense, for that was unaltered. The sense of the first phrase is thus, "no longer a servant only."

Brother ... beloved ... Any person obeying the gospel of Christ becomes the brother beloved of every other Christian, to whom all the rights, honors, privileges and love of Christian fellowship accrue as a right derived from their being "in the Lord"; and this is the heart of the great ethic which Paul here hurled in the face of a'slave-owner. The institution of slavery would in time wither and fade away under the impact of such a concept as this. The apostle doubtless foresaw this; and yet, as Lenski said, "We fail to find the least hint here that Philemon ought to set Onesimus free."[31] Some, of course, do find such a hint. See under Philemon 1:1:21.