The NT teaching that Christians are “slaves” of Christ is a metaphor full of meaning. Jesus owns us because he’s purchased us with his blood. He’s our Lord and Master because he owns us. We have a duty to trust him and obey him because we are not our own. In body and soul, God’s people are owned by Christ and now called to live – body and soul – for his glory. This is a great biblical theme!
If you want a good book on the details of being a servant of Jesus, you’ll have to get this one: Slave of Christ: A New Testament Metaphor for Total Devotion to Christ by Murray J. Harris. I don’t have time to give a full review now; I hope to do so in the future. Here’s part of a paragraph I thought was helpful in thinking of 1 Cor. 6:20 (and 7:23):
“…The body is meant to honour the Lord (v. 13); resurrection involves the body (v. 14); the body is a member of Christ (v. 15); physical unity with a prostitute compromises spiritual unity with the Lord (vv. 16-17); sexual immorality is a sin against one’s own body (v. 18); the body is the temple of the Holy Spirit (v. 19); as purchased property, Christians do not belong to themselves (vv. 19-20). “
“In establishing this latter point, Paul introduces a slogan of his own, ‘You were bought at a price.’ Neither the purchaser nor the price is mentioned, which suggests that this was a stock phrase of Pauline theology. There can be no doubt about the identity of the purchaser, for Christ is the subject of the compound verb exagorazo (‘buy out,’ ‘redeem’) in Galatians 3:13 and 4:5. As for the price paid, it is clearly the blood of Christ, given the close parallels in Revelation 5:9 (‘with your blood you purchased [egorasas] people for God’) and 1 Peter 1:18-19 (‘you were redeemed… with the precious blood of Christ’). The movement of Paul’s thought seems to be: ‘You do not belong to yourselves, but are the exclusive property of Christ, for (gar) he purchased you at the price of his blood.’ That is to say, purchase gives the right to possession; ownership is the corollary of purchase” (p. 110-120).
Indeed, it is a blessed thing – and an honor! – to be a slave of Christ. I will gladly follow and obey him because he rescued me, loves me, and watches over me. Like the Heidelberg Catechism says, “I am not my own, but belong, body and soul, in life and in death, to my faithful Savior Jesus Christ….”