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To the Apostle Paul there are only two categories of people that really matter: in Adam or in Christ. This is a major theme of the first six chapters of Romans (especially 5 & 6). Being in Adam means being a slave of sin, but one who is in Christ is a slave of God. Paul's metaphor of slavery doesn't negate human freedom, but the freedom that comes from being in Adam ultimately proves illusory and leads to death. True freedom, that leads to life, only comes through being in Christ. Romans 6:23 is a concise summary statement of this dichotomy. The Reformation Study Bible notes that:

The triple contrast of wages, sin and death, with gift, God, and eternal life, brings Paul's argument to a memorable focus.

Here's another way to picture it:

I am receiving: wages or a free gift

I am serving: sin or God

My destiny is: death or eternal life