The provision of salvation for sinners includes not only a work done in us, but also a work done for us.
What needs to be done in a sinner is to give them a new nature (‘create in me a clean heart, renew a right spirit within me’) – bringing to life the soul that was dead in trespasses and sins, and enabling the sinner to know God, to love God, and to acquiesce in God’s ways of doing things. Since these things are impossible for us to create in ourselves – just as impossible as for a child to give birth to itself – this radical change has to be brought about directly and immediately by God the Holy Spirit if it’s to happen at all. Then, after conversion/regeneration, his work in the soul is ongoing – increasing the saved sinner’s love for God and trust in God and strengthening them in habits of holiness. Those who are being sanctified die more and more to sin, and live more and more to righteousness – the path of the just is like the shining light, which shines more and more until the perfect day.
But this can’t be all that the work of salvation needs to accomplish. Because in addition to the internal corruption and defilement and deadness which the work of God in the soul deals with, there is also the matter of guilt and condemnation. That’s not just feelings of guilt, but actual guiltiness before the law, being deserving of real punishment, and condemned to that punishment which our sins deserve. These things require the sinner to be pardoned and made every bit as righteous as the law demands, while meeting the demands for satisfaction made by the broken law and God’s offended holiness.
The mechanism for achieving this aspect of salvation is Christ’s work for his people. God pardons and accepts sinners on the basis of Christ’s work for them (rather than the Holy Spirit’s work in them) – Christ’s work being that of acting as his people’s representative, bearing their sins and the punishment of their sins, fulfilling everything that the law required from them and honouring it in their place. He achieved not only the undoing of what Adam did, in other words, but also the fulfillment of what the law positively requires. The righteousness which he worked out in his life and death on this earth is not his essential righteousness which belongs to him as God the eternal Son, but what he accomplished in his role as the mediator of his own people. Whatever he achieved here in his sufferings and death, he achieved for his people – with them in mind, on their behalf, for their benefit, and in their place.
When God imputes this righteousness to particular sinners, this is the crediting to their account of what was done with them in mind to start with. It isn’t a fiction, not a question of pretending they’re something that they’re not. It brings about a change in the status of the sinner – they are no longer condemned, they are brought into a right relationship with God, his wrath is turned away from them, their sins are pardoned, they can safely be recognised as being as righteous as the law requires them to be – these are real changes, real benefits. Additionally the change in the sinner’s status isn’t effected in isolation from the change in their nature – the imputation of Christ’s righteousness, the conversion/regeneration of the soul, and the onset of sanctification are not only all instantaneous but simultaneous.
And they are simultaneous also with faith’s personal appropriation of Christ’s righteousness. The existence of a mediator and the availability of a righteousness which would suit my needs in view of the demands of God’s law are no use to me unless I get a hold of it for myself – which is what faith does, like an open hand stretched out for such blessings to be put into it. We say that God treats sinners as if this was their own righteousness, because it’s always Christ’s righteousness that belongs to him personally – but it is also true to say that this righteousness becomes their righteousness, because they take hold of Christ and his righteousness to be their own by faith.
In some ways, I suppose, because justification is a one-off legal transaction which brings about a change in the relation between sinners and the Saviour, it’s a bit more invisible than sanctification and less easy to observe in yourself or other people. Psychologically, perhaps something like effectual calling is more perceptible, or is more amenable to being latched on to on the personal level. But because justification is the only thing that does bring about that fundamental and desperately necessary change in the relation between sinners and their God and Saviour, a clear appreciation of what it consists of is essential for the soul’s wellbeing, in terms of a sense of peace and security, insight into the blessedness involved in the salvation God provides, and motivation to live soberly, righteously, and godly in this present world.