An excerpt from "Got Questions.org" entitled, "What Does it Mean to be a Slave to Sin?" states: "Everyone is a slave in the spiritual sense. We are either slaves to sin, which is our natural state, or we are slaves to Christ. The writers of the New Testament willingly declared their status as slaves of Christ. Paul opens his letter to the Romans by referring to himself as a “slave of Jesus Christ” (Romans 1:1) and his letter to Titus by calling himself a “slave of God” (Titus 1:1). James opens his epistle the same way, “James, a slave of God and of the Lord Jesus Christ” (James 1:1). Most translations say “servant” or “bond-servant” in these passages, but the Greek word doulas means, literally, “slave.”
In John 8:34 Jesus tells the unbelieving Pharisees, “Truly, truly, I say to you, everyone who commits sin is the slave of sin.” He uses the analogy of a slave and his master to make the point that a slave obeys his master because he belongs to him. Slaves have no will of their own. They are literally in bondage to their masters. When sin is our master, we are unable to resist it. But, by the power of Christ to overcome the power of sin, “You have been set free from sin and have become slaves to righteousness” (Romans 6:18). Once we come to Christ in repentance and receive forgiveness for sin, we are empowered by the Holy Spirit who comes to live within us. It is by His power that we are able to resist sinning and become slaves of righteousness."
This article may be viewed at: What Does it Mean to be a Slave to Sin?/gotquestions.org
Reference: 1 Corinthians 7:21