"Sinners cannot obey the gospel, any more than the law, without renewal of heart."
- J.I. Packer
Contrasting Legalism and Obedience: Legalism is a person's attempt to ATTAIN or MAINTAIN salvation through his or her own works, merit, humility, sound judgement or good character. Obedience, on the other hand, is the fruit of the work of Christ in which the Holy Spirit renews the heart to which it responds to its salvation by a willing and glad obedience. A quickened heart WANTS to obey ... not in order to be saved but because it is saved.
Scriptural commands (or imperatives) do not declare our ability, but our duty; what we ought to do. However, the power to obey them comes ONLY from the One that gave the command - the Spirit uniting us to Christ.