Quoting D. A. Carson from his Crossway sponsored lecture on evangelicalism at the 2009 meeting of the Evangelical Theological Society:
Justification has huge implications for how you live. What is the opposite of justification? Non-justification? Pastorally, the opposite of justification is self-justification. Over against being justified by someone outside ourselves—being justified by God, through what he has done in Christ—we justify ourselves.
So the man, for example, who approaches Jesus: “What must I do to inherit eternal life?”, whence Jesus gives him the first round of responses, he asks another question. Luke’s comment is, “He, wanting to justify himself, said….” And then a few chapters farther on, further people approach Jesus “wanting to justify themselves.” Or the parable of the Pharisee and the publican going up to the temple together, the Pharisee saying, “I thank you God that I am not as other men are, including this wretched publican over here.” What is that but self-justification?
So now you have come out of a rotten background where you never could gain enough of your parent’s approval. They were just so harsh and miserable all of the time. And you’ve become a Christian, and you know that you’re justified before God. What is it in you, then, that is constantly trying to show yourself good enough to be accepted by others, to be loved by church people, to be accepted by your siblings? Isn’t that a form of self-justification that is denying the justification that you have experienced in the onset of the gospel?
There is so much of Christian discipleship and growth that is bound up with the cross-work in justification. What sins do we commit where we are not tripping over self-justification? Self-justification in our publications, in our schools, in how our spouses think of us, in how we think about ourselves? Self-justification, even though at some level we know we’ve been justified by another?
If the gospel is rightly understood, if the gospel is rightly conceived, the glory of being justified by God himself through what he has provided in his Son by grace alone through faith alone begins to transform all of our relationships. In one sense, sanctification, understood in the Reformed sense (not always in the Pauline sense), is nothing other than the progressive application of justification.