What is justification?
Justification is the act in which God declares sinners to be righteous. It is the language of the law courts (which is why it’s sometimes referred to as forensic, i.e. legal, justification). In the same way that the judge declares the defendant to be not guilty, so God declares believers to be righteous, that is, not guilty in his sight.

There are two important distinctions to be made:
First, God doesn’t declare us to be righteous because he perceives any righteousness in us. Not even our faith can be counted as a righteous act that puts us in good standing with God.

And second, God doesn’t make us righteous in order to justify us.

The classic text here is Romans 3:23-24
‘For all have sinned and fall short of the glory of God and are justified by his grace as a gift…’

Justification is a gracious gift from God, wholly undeserved.

On what basis does God justify believers?
If our justification is not the result in any way of our righteousness, how can it be just? It would clearly be just for God to condemn us as sinners (notice the legal language again). How can it be just for him to declare that we are not guilty or righteous?

This is where the important doctrine of union with Christ comes in. The bible teaches that when we put our faith in Christ we are united to him. I can’t remember which Puritan it is who used the image of us hanging from the belt of Adam and then being transferred to the belt of Christ. Our lives are bound up in his. Our guilt is imputed to him (that is, it is counted as his.) When he died on the cross he was able to take our punishment because he bore our guilt. In the same way, his righteousness is imputed to us (that is, it is counted as ours).

In Christ, therefore, God can declare that we are righteous - not with a righteousness of our own but with Christ’s righteousness. When God looks on us, he sees us-in-Christ, and so of course he can say with complete justice, ‘Not Guilty!’

This is what Paul is explaining in Romans 3:24-26
‘…through the redemption that is in Christ Jesus, whom God put forward as a propitiation by his blood to be received by faith. This was to show God’s righteousness, because in his divine forbearance he had passed over former sins. It was to show his righteousness at the present time, so that he might be just and the justifier of the one who has faith in Jesus.’
God is both just and the justifier. He declares us righteous and he does so justly.

How can we be justified?
So then we can see the role of faith in justification. Faith is the means by which the necessary union with Christ takes place. By faith (remembering always that faith is the gift of God and not a means of gaining credit for us), we move from being ‘in Adam’ to being ‘in Christ’. By faith, therefore, we take on Christ’s righteousness and he bears our guilt. We can say, therefore, that it is through faith that we are justified.

So Romans 3:28-30
‘For we hold that one is justified by faith apart from works of the law. Or is God the God of the Jews only? Is he not the God of Gentiles also? Yes, of Gentiles also, since God is one. He will justify the circumcised by faith and the uncircumcised through faith.’