Mainliner-
The Lord's Prayer has that question covered:
"and forgive us our trespasses, as we forgive those who trespass against us"
The question you raised is an excellent one. In the Old Testament God did hold men accountable for sins that they did not know they had committed. In Leviticus chapters 4,5, and 6 we find the sacrifices necessary for them to make once they realized that they had fallen short of the mark.
What we Christians need to understand is that our salvation is not as a result of our own righteousness; it's the direct result of God's own righteousness. We are all sinners, both before and after our acceptance of salvation. But unlike before, when we were sinners lost for eternity, we are now sinners saved by the grace of God:
Now we know that whatever the law says, it says to those who are under the law, so that every mouth may be silenced and the whole world held accountable to God. Therefore no one will be declared righteous in his sight by observing the law; rather, through the law we become conscious of sin.
But now a righteousness from God, apart from law, has been made known, to which the Law and the Prophets testify. This righteousness from God comes through faith in Jesus Christ to all who believe. There is no difference, for all have sinned and fall short of the glory of God, and are justified freely by his grace through the redemption that came by Christ Jesus. God presented him as a sacrifice of atonement, through faith in his blood. He did this to demonstrate his justice, because in his forbearance he had left the sins committed beforehand unpunished - he did it to demonstrate his justice at the present time, so as to be just and the one who justifies those who have faith in Jesus. (Romans 3:19-26,NIV)
The righteousness which God accepts as true righteousness is his own righteousness. Through his actions, and with the full cooperation of his Son Jesus Christ, he has enabled us to have this righteousness. But we must remember that the righteousness is his, given as a gift to us, not ours, earned through our own work.
We do what we can in the furtherance of God's kingdom. But we do it out of gratitude for what we have already received, namely, salvation, rather than doing it in an attempt to augment the righteousness received from God in order to attain that salvation. All we can do is say, "Thank you," through our own work for that gift, both unearned and unrepayable.