In this study the version of the Bible is a factor when you are looking for slave, servant, or bond servant; KJV renders doulos as servant while more modern translations use slave. (See the post – Slaves) Paul used the term doulosmost of the time even when referring to his relationship with God. But, Paul used several other terms that are translated servant/slave: 3610 – oiketesin Romans 14: 4 and once in Philemon; 1248 – diakoniain Romans 16: 1. He also used 3000– latreuo(to serve for hire, minister, worship) in Romans 1:9 and 1 Timothy 1:3; 1398 – douleuoin Romans 6:6, 7:6, 7:25, 9:12, 16:18; Galatians 5:13; Colossians 3:24; 1 Thessalonians 1:9. ( See Slaves, these are from a Strong’s Concordance KJV and the italic numbers are the New Testament references.)
Paul wrote to believers who were gentiles in a Greek and Roman world. Many of churches had slaves or slave holders, so in a sense some of what he was writing would have been very unusually to them.