What's the Difference Between Law and Promise?
For the promise to Abraham and his offspring that he would be heir of the world did not come through the law but through the righteousness of faith. For if it is the adherents of the law who are to be the heirs, faith is null and the promise is void. - Romans 4:13-14, ESV
It's important to known the difference between different things. In Price Caspian, Trumpkin the Dwarf says, "I know the difference between giving advice and taking orders. You've had my advice, and now it's the time for orders." As Christians, it is vitally important that we know the difference between God's law and His promise.
God's law is an expression of His will for us. It is what God requires of His people. God's moral law reflects His moral character, and it is binding on all people. He has made all of us in His image, and we are called to reflect His character. However, sin has broken us at our core, and we are unable to keep God's law.
Our salvation is not found in the Law, which is God's requirement of our conduct. It is found in God's promise, which is not what He requires us to be and do, but what He has promised that He will be and do. We break God's law in thought, word and deed daily, by what we choose to do and by what we leave undone. God never breaks His promise.
The right response to God's law is obedience, which we should always strive to give, but which we know we never give perfectly. The right response to promise is faith. It would be as inappropriate to respond to God's promise with works as it would be to respond to His law with a passive profession of faith that did not strive after obedience.
Think about it: If I ask you to do something, I want you to responding by doing what I'm asking you to do. But if I promise to do something for you, I don't want you to try to do it. I want you to trust that I am going to do what I promise.
God has promised to save us through Jesus Christ. Jesus has done all the work to save us, and God promises to save us if we trust in Jesus. To try to earn that salvation by doing works is to seek to replace Jesus' work with our own. In that case, good works don't flow from faith but seek to be a replacement for faith. A promise is to believed and received, and not to be earned.