As spiritual beings (that is, creatures who have a sense of transcendence beyond ourselves) our spirit, or our internal guidance system, is tuned in to the "prince of the power of the air" according to Ephesians 2:1-3. This is essentially a form of slavery to the desires that the evil one plants into our hearts, usually in the form of self-centeredness. We are simply under the illusion that this is our free will because it tends to satisfy our selfish desires. God offers to take us out of this spiritual realm and to tune us in to to Spirit of Christ who comes to dwell within us when we receive Him. Jesus then becomes our internal guidance system, shaping our desires and motivations to serve Him and experience the joy and peace in the Spirit that He has for us.

Think you're not a slave? You're a slave to every desire that drives you. You're simply unaware of who planted those desires within you.
@ C.D.M. I think you missed something. The same gospel that gives us a clean slate also gives us a clean heart. It's called imputed righteousness AND imparted righteousness. Perhaps you need to spend some time reading Romans 6, 8 and Romans 12:1, 2. The gospel does not teach us that we are to freely give in to sin because God will freely forgive our sin. That is a perversion of the gospel. Certainly we are forgiven when we sin, but we have also been given the power to overcome sin's power over us.