One of the central issues of the Reformation is also a central issues of human eternal destiny: How does one get right with God?
In theological terms, this is called "justification." It's an ancient legal, or forensic, term that carries a very special meaning in the Bible. Our legal system really has no parallel for it. "acquitted," or "not guilty" comes close but doesn't quite capture it.
The situation that is envisaged in the Bible really is a mixture of what we would call legal and civil proceedings. A legal proceeding is one wherein a law has been broken, and the alleged perpetrator is before judge or jury to determine guilt or innocence. In a civil proceeding, the judge chooses between two parties, one who has been harmed in some way, and the one who has allegedly done the harm.
According to the Bible we are judged by God for our sins, which are at the same time matters of breaking God's Law, and a matters of causing harm to God. One trial, one judgment, speaks to both the "judicial" and "civil" elements.
It would be, in our society, a gross miscarriage of justice for the victim to be the judge in either case, let alone both. We would assume impartiality to be impossible, and that this "justice" is little more than vigilantism.
What then are we to say when it is made clear to us that through faith we are justified by God? As I said earlier, the words we typically apply, "acquitted" and "not guilty," are insufficient. Those terms suggest that we in ourselves are found to be innocent. That isn't the case; if it were then no one would be punished. Yet Christ died for our sins. Clearly in the matter of our sins, we are found guilty. The reason we are not punished though is Christ Jesus. He took our penalty upon Himself. In that process we received his righteousness, and that is stronger than just innocence. Innocence connotes the mere absence of evil, but righteousness gives the idea of having been obedient.
That's what happens when we believe in Jesus Christ. When we trust Him to bear our punishment for us, we find that He did just that. We also find that God declares that we have not been disobedient, but we have been actively obedient to His will, in all ways, and in all things. Our obedience is found to be perfect, because that's the obedience that Jesus had.
Now since that obedience is perfect, there is nothing we can do to add to it. Nothing at all. And it doesn't matter if our obedience from the point of belief onward is not perfect; all our failures were born by Jesus on the Cross, even the ones that follow that moment of faith.
That doesn't mean though that we can just live however and get away with it. The fact is that when you are justified, your whole life orientation changes; you want to serve and please God. Your lifestyle will reflect that inevitably. There is no going back to the old way because you don't want to go back. Sure, there may be moments when you feel like it, but over the long run your life will be characterized by obedience to God, not sin.
So it happens that when the Final Judgment comes, God can say what he already said; you are justified, righteous. He said you were when you believed, and because of that, your life since gives evidence of it.
This is how the Reformation has come to understand the answer to the question: How do you get to be right with God? I confess I don't understand why so many don't believe when they have it explained to them. I don't understand why so many who are Christians don't accept it, but would rather try to add to Jesus' obedience with some of their own.
Maybe its just because I've studied the question so long. Maybe.