Romans 8:2 For the law of the Spirit of life in Christ Jesus hath made me free from the law of sin and death.

The law of the Spirit, is the commandment to believe in Christ, for through faith a person takes possession of the Spirit of promise, in the form of the earnest of the inheritance. Observance of the law of the Spirit, is to follow, obey, and conform to the guidance of the Spirit, not grieving or quenching it. Looking forward to Romans 8:5,  it is to think like the Spirit, always attentive to the things of God.

To believe, our faith must include the observance of the law, or else our faith is dead, worthless, and unfruitful. Yet, by our faith, we are freed from the law. This is true, because faith is not produced, through the observance of the law, but rather, the law is observed through faith. The works of the law are burdensome, when performing them in order to obtain a reward. But, if we have confidence that we have already obtained the greatest reward, and observe the law out of love, respect, honor, and gratitude, then the law is obeserved without effort or desire, and the burden is almost nonexistant (Matthew 11:30). And for doing that which seems to be effortless, God is faithful to reward.

Romans 4:4 Now to him that worketh is the reward not reckoned of grace, but of debt.