The illustration of the accounting ledger is usually presented to Christian people in terms of a debt to owe, but that God pays the debt, bringing a person from a negative to a positive. The picture is that because of our sin, we have a lack of righteousness, and that the righteousness of Jesus, earned by means of his life on earth, is "imputed" or "written in" on the plus-side of the ledger. We've gone from negative to positive, and we now have the right to eternal life. This illustration is wrong, however.
It is wrong, not because it is not true that people are in debt to God. We are. We have sinned against Him and we must pay the penalty of death. That is, we must pay the penalty if no other payment has been paid on our behalf. Jesus Christ has indeed paid that penalty by means of his own death (and resurrection), but the "accounting ledger" picture is still amiss. It is so because it presupposes a system of merit with God, and God does not and never has dealt with humans by means of merit. The Westminster Confession of Faith (chapter 19, point 1), speaks of a "covenant of works" with Adam, where Adam "was promised life upon the fulfilling" of the covenant of works. So, Adam had to earn his right to eternal life, as if he did not have it to begin with. How can this be?
This is how the idea of merit has entered into Reformed thought. But the Lord has never, does not, and will not ever deal with his creation on the system of merit. This is why the accounting ledger illustration misleads people, making them think that God "cancels debt and puts money in the bank," where the "money" is a "positive righteousness" which allows someone to "get into heaven."
A better way to think of justification is simply this: the forgiveness of sins. In Romans 4:25, Paul writes that "He [Jesus] was put to death for our sins and raised to life for our justification." Paul here says that justification is the forgiveness of sins, by linking the death of Jesus for our sins with his resurrection from the dead. It is the resurrection of Jesus that brings life to us by his victory over the grave. Our faith in him unites us to him in life resurrection life (Romans 6:1-14).
In chapter 5, Paul says, "and the free gift is not like the result of that one man's sin. For the judgment following one trespass brought condemnation, but the free gift following many trespasses brought justification (v.16). Again, Paul unites together the contrasting ideas of condemnation of death due to sin and the right to eternal life due to the "free gift" (the death and resurrection of Jesus) which brings justification--or, the forgiveness of sins. It is the forgiveness of sins that brings life, not the -/+ conceptual scheme of the accounting ledger. In the same way, Paul argues in v. 18 of chapter 5: Therefore, as one trespass led to condemnation for all men, so one act of righteousness leads to justification and life for all men.
Now, surely someone will say, "Look at verse 18 which you just quoted. How can it be that the accounting ledger does not describe perfectly what is happening here? See: the negative debt we owe is death and condemnation, while the positive gift we receive is justification and life. The accounting ledger is a perfect picture of this." It may seem like that on the surface, but the accounting ledger is in three stages, actually: the first is to cancel one's debt. This is the first stage. Then, this brings a person to the state of neutrality. This is the second stage. The third stage then, is to give someone the positive righteousness they need. This is the third stage. The picture of the Apostle Paul is much simpler, and it doesn't rest on a works-righteousness way of thinking.
Rather, it is simply two stages which occur in one, fell swoop: we are sinners under the condemnation of God who receive the free gift of God's grace which brings the forgiveness of sins and the right to eternal life. There is no system of works here: Jesus didn't earn righteousness for us so he could pay the positive side of the ledger after he had paid the negative side of the ledger with his death. On the contrary, both his death and resurrection bring justification and life to the one who has faith in Jesus.
The accounting ledger illustration unfortunately reinforces the "works" concept, a concept that never entered the mind of God. God did not deal with Adam according to works, and he doesn't deal with us that way, either.