Many people are confused about the relationship between faith and works. This confusion can have serious consequences since an erring view of their relationship could lead someone into believing and teaching something that is very unbiblical such as the heresy that we are saved by cooperating with God by doing good works.
First of all, justification is the legal declaration by God upon the sinner, where God declares the sinner to be righteous. This declaration is based completely and totally on the work of Christ on the Cross. A person is justified by faith (Rom. 5:1); that is, he is made right before God by his faith in Christ (excluding the cults that teach a false Jesus). We are not saved by our works or our works added to the sacrifice of Christ. Our works, our good deeds, have absolutely no affect upon our salvation. Our good works do not get us salvation, nor do they help us keep our salvation. This is because our good works are filthy rags before God (Is. 64:6). Besides, if we could be saved by works, then righteousness would have been based on the law and Christ would not have needed to die. Gal. 2:21 says, "I do not nullify the grace of God; for if righteousness comes through the Law, then Christ died needlessly." But, Christ did need to die which proves our works cannot save us.
However, this does not mean that we are not to have good works. The Bible clearly tells us that if we claim to be Christian and we do not have good works, then we are not saved (1 John 2:4). Furthermore, the Bible also says that once we are saved, we are not free to be bad (Rom. 6:1-2). On the contrary, we are obligated to be good. In John 14:15 Jesus says, "If you love Me, you will keep My commandments." Also, 1 John 2:3 says, "And by this we know that we have come to know Him, if we keep His commandments."
Confused? Don't be. We are not saved by our works because we cannot be. The only way to be saved is to trust in the finished sacrificial work of Christ on the cross. But, once saved, the person is now changed. He is a new creation and the old things have passed away (2 Cor. 5:17). This means that our old sinful ways are gone (or on their way out as we struggle against them). It also means that we then do good works to honor God. We don't do good works to be saved or stay saved. We do good works because we are saved. I once heard it said that you are not saved by good works, but you aren't saved without them. This means that our good works don't save us in any way, but once we are saved we naturally obey God's law and do good works which is a sign of our being saved.
So the relationship between faith and works is simple. We are saved by faith, not by works. But, once saved, we do good works because we are already saved.