For therein is the righteousness of God revealed from faith to faith: as it is written, The just shall live by faith. Rom. 1:17.
Obedience comes only by faith. Obedience is by faith alone. Those who have been counted just shall live by faith as well. One of the reasons that obedience can come only by faith, and by faith only (and if you don't think I'm trying to say that more than once, you've missed the point), is found in the Sermon on the Mount. There Jesus teaches that you don't have to commit adultery to commit adultery. And you don't have to kill someone to be guilty of murder. If you have lust or anger in your heart, you are guilty already.
If this is true, then much of what we would like to call obedience is actually sin. If I want to cheat on my income tax, but I don't do it, I am still a cheat. If I find the idea of robbing a bank appealing, even if I have never stolen a dime in my life, I'm still a thief. In fact, if I crave unhealthful foods, even if I am strong-willed enough to refuse to eat them, I can still be a glutton! Notice this comment in Councils on Diet and Foods, page 35: "Men will never be truly temperate until the grace of Christ is an abiding principle in the heart. All the pledges in the world will not make you or your wife health reformers. No mere restriction of your diet will cure your diseased appetite....Christianity proposes a reformation in the heart. What Christ works within will be worked out under the dictation of converted intellect. The plan of beginning outside and trying to work inwardly has always failed, and always will fail. God's plan with you is to begin at the very seat of all difficulties, the heart, and then from out of the heart will issue the principles of righteousness."
Let me ask you a question. If God has wrought His work in your heart so that you have begun to see sin as He sees it; so that your tastes, inclinations, ambitions, and passions are brought under His control (Selected Messages, book 1, p. 336); so that your feelings, thoughts, purposes, and actions are in harmony with God's will (Steps to Christ, p. 61)--if that has happened, will you have to try hard to obey? Why, you'd have to try hard not to! Through the ongoing faith relationship with Jesus, we can be changed into His image, so that we are obedient--from the inside out.