Neither yield ye your members as instruments of unrighteousness unto sin: but yield yourselves unto God, as those that are alive from the dead, and your members as instruments of righteousness unto God. Rom. 6:13.
An instrument is something that is used and controlled by the artisan or the soldier. An ax in the hands of a 4-year-old will not bring down a giant tree in the forest. But an ax in the hands of an experienced woodsman will bring down the tree. The law, being weak through the flesh, is never going to be kept by people apart from Christ. But when a person becomes an instrument in the hands of Christ, then obedience is possible.
An instrument is a passive thing, but many of us are afraid of the word passive. Someone asks, "Do you mean that I am to yield up my will to God, and be controlled by Him, so that I won't even choose what color of wallpaper to put on the wall? I won't even choose what color of socks to wear, or what dress to put on?" Well, I'd like to ask you a question: What's so frightening about that? You know, I have chosen the wrong wallpaper a few times. I would just as soon have a little help with the wallpaper! Especially with hanging it! But if you don't want to go that far, then go at least this far: We are absolutely dependent upon God's control for obedience to His law.
We don't have to be afraid of becoming puppets, of losing our power of choice, or our individuality. If it is the Creator who invites us to come under His control, why does the matter seem so scary? Our Creator is the One who wanted us in His image, with individuality and power of choice, to begin with. The idea of making us "free moral agents" was His in the first place. Can't we safely leave the rest of the road in His hands, and respond to His invitation to yield ourselves to His control?
My son used to like to jump off the piano bench into my arms. It wasn't his idea at first. I thought it up! I put him up there and said, "Jump. I'll catch you." Without a moment's hesitation, he jumped. And I caught him. Why? Because he trusted me. If I trust my heavenly Father, and He invites me to come under His control, to surrender myself to Him, will I not be safe in His hands? Trusting Him is the only kind of real freedom there is.