Slaves are free when they are slaves of a perfectly loving Master. If you are a Christian; if you are one who has been born of God’s Spirit and are on your way to the New Jerusalem, then you are a slave of Christ Jesus. This kind of slavery is the freest of all, and is the very thing we were created for. Sound like an ugly idea to you? Then you must learn Christ better! He is the worthy Master, the Master who loved us so that He gave His life for His slaves… even redefining slave as being “friends” (John 15:15).
R.C. Sproul gives us the details on being slaves of Christ, commenting on Paul’s introduction of himself in Romans 1:1 –
The Greek word Paul used here is doulos. A doulos was not a hired servant who could come and go as he pleased. A doulos was a person who had been purchased, and once purchased he became his master’s possession.
The idea of the doulos in Scripture is always connected to another descriptive word, kurios . . . The supreme use of kurios [in the New Testament] refers to the sovereign God, who rules all things. Kurios, “the name which is above every name” (Phil. 2:9), is the name given to Jesus, whom the Father calls the King of kings and the Lord of lords. There is yet a middle usage of the term kurios in the New Testament. It is used to describe a slave owner, which is an apt description of Jesus, and it is from this that Paul describes himself. He is not just a servant but a slave.
Paul, in addressing believers, said, “You are not your own. For you were bought at a price” (1 Cor. 6:19). We have been purchased by the blood of Jesus Christ (Acts 20:28). There is a paradox here: when the New Testament describes our condition by nature, as fallen people, it describes us as slaves to sin . We are by nature in bondage to sin, bondservants of the flesh, and the only remedy for that, according to the New Testament, is to be liberated by the work of the Holy Spirit. For “where the Spirit of the Lord is, there is liberty” (2 Cor. 3:17). Everyone born of the Spirit is set free from slavery to sin.
There is also irony here: when Christ sets us free from slavery to the flesh, he calls us to the royal liberty of slavery to him. That is why we call him Master. We acknowledge that it is from him that we get our marching orders. He is the Lord of our lives. We are not our own. We are not autonomous or independent. Unless people understand their relationship to Christ in these terms, they remain unconverted.
Is Jesus your Master? In other words, are you free?
 R.C. Sproul, Romans (Wheaton, IL: Crossway, 2009), 16-17.