Whenever you read that word “servant” in the New Testament, you need to remember what it really means. Strictly speaking, in ancient times they did not have servants—at least not in the sense that we have in mind when we use that word. Rather, what they had were slaves. So when you read in the New Testament about somebody being a servant, generally that word translates the Greek word for slave, which is doulos. Thus, this is what we would expect Paul to be saying here. He considers himself as a slave of Christ.