“We know that our old self was crucified with him in order that the body of sin might be brought to nothing, so that we would no longer be enslaved to sin.” -Romans 6:6
Using an old man/new man narrative, Paul now begins to expound on the theological mechanics of that which he asserted in response to his hypothetical interlocutor’s question: What shall we say then? Are we to continue in sin that grace may abound? and his more concise statement made in verse 5 about sharing in resurrection of Christ also because we shared in the death of Christ. In other words, Paul now explains to his Roman audience how the union to Christ works.
The expression our old self and body of sin don’t refer to the physical body but to our sinful humanity, our fallen condition brought about due to our sonship to the first Adam and subsequent sinful choices. Those who have faith in God’s promise are grafted into Christ by the Spirit so that we might be crucified with him. It is through our crucifixion with Christ that our old self, our body of sin is destroyed.
That being the case, we are liberated from the power of sin over us. We are no longer enslaved to it. Sin is still present but it is no longer a master over us.