1 Corinthians 7:21-22

21Were you a slave when you were called? Do not worry about that [since your status as a believer is equal to that of a freeborn believer]; but if you are able to gain your freedom, do that.

22For he who was a slave when he was called in the Lord is a freedman of the Lord, likewise he who was free when he was called is a slave of Christ.