John Chrysostom

Next he adds also the cause; "For he that was called in the Lord being a bondservant, is the Lord's free man: likewise he that was called, being free, is Christ's bondservant." "For," saith he, "in the things that relate to Christ, both are equal: and like as thou art the slave of Christ, so also is thy master. How then is the slave a free man? Because He has freed thee not only from sin, but also from outward slavery while continuing a slave. …

Homily on 1 Corinthians 19 ↗

Theophylact of Ohrid

A freedman is one who has been liberated from slavery. So he says: you who believed while in a state of slavery are a freedman of the Lord, for Christ has freed you both from sin and from that outward slavery, even though you are a slave. He who does not submit to passions, having a noble soul, is not a slave, even though he appears to be one. On the other hand, another was called to the faith as a free man; such a one is a slave of Christ. …

Commentary on 1 Corinthians ↗