But what does it mean to be justified, and why is it such good news? Martyn Lloyd-Jones puts it better than I could:
But this is the amazing message, and this is what is meant by justification – that God tells us that, as the result of the work of the Lord Jesus Christ, because of his life, his death and his resurrection, if we believe on him and trust ourselves solely and entirely to him, God pardons and forgives our sins. Not only that, he declares that we are free from guilt: more than that, justification includes this. He not only declares that we are pardoned and forgiven and that we are guiltless, he also declares that we are positively righteous. He imputes to us, that is, he puts to our account, the righteousness of the Lord Jesus Christ himself, who was entirely without sin, who never failed his Father in any way, and who never broke a Commandment or transgressed any law. God gives to us – puts upon us – the righteousness of the Lord Jesus Christ himself, and then looks upon us and pronounces that we are righteous in his holy sight. That is the biblical doctrine of justification.