"All that is not of faith is sin." — But the preceding context is "if you have doubts about whether or not you should …". The NLT says it more understandably as: "If you do anything you believe is not right, you are sinning.". That may be true, but it is not "the definition of sin in the Christian Bible". A better definition of sin is in 1 John 3:4, "Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law.". The law is a reflection of how God behaves, which is how all should aim to behave, so sin is behaving in a way that God would not behave.

@RayButterworth The Giver of the Law is not ruled by Law, for that would make God obliged to it, subject to it. God IS righteous by nature of his Being; we are not. The Law given to Moses is perfect, designed to convict all who try, and who all fail to keep it, that they are sinners. The Law condemns us all, so that we flee to Christ to be transformed by grace. The principle of "All that is not of faith is sin" can be applied to everything in life. Those with God-cleansed consciences behave so as to please God, applying faith to all that they do and say, enabled by the indwelling Spirit.

And, given your last sentence, we would remain mired in sin for we all taint our every behaviour with sin. So the scriptures state “For the law is not of faith”, “not having my own righteousness, which is of the law”, “I through the law am dead to the law that I might live unto God”, “For by the law is the knowledge of sin”, “The righteousness of God without the law is manifested”, “now delivered from the law… serve not in the oldness of the letter”,“The ministry of the law brought death”, “If there had been a law given that could have given life, righteousness should have been by the law” etc

"The Giver of the Law is not ruled by Law, for that would make God obliged to it, subject to it.". — Suppose you run a business, and you post rules that describe how you do the things your employees now do for you, believing yours is the best way. You want everyone to do it like that and you will fire anyone that doesn't follow your rules (though you will forgive those that admit error and change). ¶You yourself will follow the rules, but not because you are obliged to or are under the rules; you are the rules. And you are not subject to the rules; you are the subject of the rules.

@Anne asks "Why cling on to it when Christ died to liberate us from bondage to legalism?" — Not bondage, but freedom. The goal is to think and behave as God does, naturally following the law, not because of obligation to obey it, but because being like God has become one's natural behaviour. The law simply describes what God is, and what one can become. ¶Biblical analogy: "thou shalt not steal" — OT: stealing is illegal and will result in a penalty. NT: God doesn't steal, why would I even think of doing so myself? Each view results in following the law, but one is bondage the other freedom.

Ray, Christ set us free from bondage to the handwritten documents that were contrary to us, that were against us, by taking it away, nailing it to the cross (Col.2:14). That is Christian freedom. Now, why would anyone so liberated then turn around and walk back to the law

(if only one-third of it, the 'Moral law') insisting this still has to be carried out, even though they still cannot keep it? Then they say that even when they do break it, there's no punishment for that. What? A law with no punishment? A toothless recommendation more like! That's not the law of God at all, if there's no power to punish those who break it!