I jumped at the chance to get the new John MacArthur book and I was not disappointed. "Slave" is an enlightening treasure, making sense of our relationship to Christ. John MacArthur discovered through study that most modern translations of the Bible, KJV included, use the word "servant" in place of "slave", entirely altering the meaning of the text. Before we come to Christ, we aren't servants of sin, we are slaves to it. When Christ saves us, we become slaves of righteousness. (Romans 6:16-20) We belong to Christ and as such, we lovingly submit to Him as Master.
Our status as slaves, and not merely servants, has serious implications for how we think and act as believers. He has exclusive ownership and we are completely dependent on Him and accountable to Him. Unlike worldly masters, Jesus supplies all our needs, strengthens us to do His will, and gives us grace, sufficient to accomplish His purposes.